BRARY 

IVMSITYOI 


A  DETAILED   COURSE 


OF 


QUALITATIVE 
CHEMICAL  ANALYSIS 

OF    INORGANIC   SUBSTANCES, 

WITH 

EXPLANATORY  NOTES. 

B¥ 

ARTHUR   A.    NOYES,    PH.D., 

PROFESSOR    OF    CHEMISTRY    IN    THE    MASSACHUSETTS 
INSTITUTE    OF    TECHNOLOGY. 


THIRD    REVISED    AND    ENLARGED    EDITION. 


NEW    YORK: 
THE    MACMILLAN    COMPANY. 

LONDON  :    MACMILLAN  &  Co.,  Ltd. 
1899, 


COPYRIGHT,  1897, 
BY    ARTHUR    A.    No  YES. 


PREFACE. 


SOME  explanation  of  the  origin  and  purpose  of  the  present  work 
*s  necessary.  It  arose  out  of  the  difficulty  experienced  in  attempt- 
ing to  give  a  thorough  course  of  qualitative  analysis  in  limited  time 
to  large  classes  of  students.  The  smaller  text-books  on  the  sub- 
ject are  too  elementary  in  character  to  use  in  connection  with  such 
a  course ;  they  devote  too  little  attention  to  the  precautions  to  be 
observed  to  insure  delicacy  of  the  reactions,  to  the  difficulties  en- 
countered by  the  student  in  actual  work,  and  to  the  modifications 
of  the  process  required  in  special  cases.  On  the  other  hand,  the 
manual  of  Fresenius,  though  complete  and  very  valuable  as  a  work 
of  reference,  is  confusing  to  beginners  on  account  of  the  complex- 
ity of  its  arrangement  and  insufficient  explanation  of  the  purposes 
of  the  various  operations.  The  attempt  has  been  made  in  the  fol- 
lowing pages  to  present  an  almost  equally  thorough  scheme  of 
qualitative  analysis  in  a  more  concise  and  readily  intelligible  form. 

The  process  of  analysis  here  given  was  based  originally  on  that 
of  Fresenius,  but  it  has  undergone  a  number  of  important  modifi- 
cations suggested  by  experience  with  it  in  the  laboratory.  Among 
these  may  be  specially  mentioned  the  directions  for  the  prepara 
tion  of  the  substance  for  analysis  and  those  for  testing  for  acids, 
which  have  been  made  as  distinct  and  definite  as  possible  ;  for  it 
is  with  these  two  parts  of  the  process  that  the  greatest  difficulty 
is  usually  experienced.  The  methods  of  separation  of  the  metals 
also  differ  somewhat  from  those  of  Fresenius,  a  different  process 
being  given  for  arsenic,  antimony,  and  tin,  and  the  choice  between 


4  PREFACE. 

the   two   methods   of  analysis   of  the  aluminum  and  iron  groups 
being  made  to  depend  on  a  preliminary  test  for  phosphates. 

The  most  characteristic  feature  of  the  book,  however,  will  be 
seen  to  be  the  method  of  presentation  of  the  subject.  Through- 
out the  part  devoted  to  the  description  of  the  process  of  analysis 
the  matter  is  comprised  under  two  distinct  headings  — the  method 
of  Procedure  and  the  Notes  upon  it.  The  former  consists  exclu- 
sively of  very  detailed  directions  for  carrying  out  the  separations 
and  special  tests.  The  latter  serve  :  first,  to  criticise  the  process 
and  to  point  out  the  conditions  to  be  fulfilled  in  order  to  insure 
delicacy  of  the  tests ;  second,  to  show  the  purpose  of  each  oper- 
ation and  reagent,  where  this  is  not  sufficiently  obvious  ; .  third, 
to  explain  abnormal  results  arising  either  from  previous  errors  in 
the  analysis  or  from  the  existence  of  conditions  not  provided  for 
in  the  general  scheme  ;  and  fourth,  to  suggest  modifications  of  the 
process  desirable  in  special  cases.  In  the  part  devoted  to  the 
analysis  for  metals  the  description  of  each  group  is  preceded  by  a 
brief  tabular  outline  of  the  process,  intended  only  to  indicate  the 
principles  of  the  separation,  not  to  serve  as  a  working  basis. 

Besides  this  description  of  the  process  of  analysis  the  book  con- 
tains a  separate  part  giving  the  Directions  for  Laboratory  Work 
which  have  been  used  in  connection  with  it.  The  author  desires 
to  express  the  opinion  that  the  plan  followed  here  and  in  the 
manual  of  Eliot  and  Storer  of  analyzing  by  the  regular  process 
solutions  known  to  contain  all  the  metals  of  each  group  is  a  far 
better  preparation  for  the  subsequent  analysis  of  unknown  sub- 
stances than  the  more  usual  method  of  studying  the  reactions  of 
each  metal  separately ;  for,  however  valuable  the  knowledge  of 
the  additional  reactions  involved  in  the  latter  method  may  ber 
it  is  found  in  practice  that  the  performance  of  such  a  large  num- 
ber of  independent  tests  in  disconnected  form  makes  but  little 
impression  on  the  mind  of  the  student  and  causes  confusion  and 
loss  of  interest.  Even  where  it  is  desired  to  teach  the  chemistry 
of  the  metals  in  connection  with  a  course  of  qualitative  analysis 


PREFACE.  5 

it  is  in  the  author's  opinion  better  to  do  so  as  a  supplement  to  the 
scheme  of  separation,  in  the  form  of  additional  reactions  of  the 
various  metals.  The  laboratory  work  should  be  accompanied  by 
lectures  or  recitations  in  which  the  process  of  analysis  is  taken 
up  and  discussed  in  detail. 

In  conclusion,  the  author  desires  to  express  his  great  indebt- 
edness to  Prof.  T.  M.  Drown  for  his  encouragement  and  valuable 
advice  in  regard  to  the  general  features  of  the  book,  and  to 
Mr.  W.  S.  Davenport,  Instructor  in  Qualitative  Analysis  at  the 
Institute,  through  whose  cordial  cooperation  and  numerous  sug- 
gestions it  has  been  greatly  improved,  not  only  in  general  char- 
acter, but  also  in  matters  of  detail. 


PREFACE  TO  THE  THIRD  EDITION. 


IN  this  edition  a  number  of  changes  suggested  by  recent  inves- 
tigations and  by  the  continued  use  of  the  book  in  the  laboratory 
have  been  made.  In  accordance  with  the  researches  of  Fresenius, 
a  much  more  delicate  method  of  detection  of  the  alkaline-earth 
metals  has  been  introduced.  The  barium  carbonate  process  in 
the  presence  of  alkaline-earth  phosphates  has  been  simplified. 
The  methods  of  detection  of  metals  and  acids  in  the  dry  way 
have  been  extended  and  more  fully  discussed ;  and  practice  with 
them  has  been  introduced  into  the  Directions  for  Laboratory 
Work.  On  the  other  hand,  in  the  case  of  complete  analyses,  it  is 
directed  to  confine  the  preliminary  dry  examination  to  the  closed- 
tube  test  —  a  modification  discussed  on  page  65.  Finally,  for  the 
convenience  of  teachers,  an  appendix  has  been  added  showing  the 
strength  of  reagents  to  be  employed. 

Objection  has  been  raised  to  the  system  of  instruction  fol- 
lowed in  this  book  on  the  ground  that  the  omission  of  prelimi- 
nary "  reaction  "  work  gives  the  student  too  narrow  and  limited  an 


6  PREFACE    TO    THE    THIRD   EDITION. 

acquaintance  with  chemical  facts.  In  reply  to  this  objection,  and 
especially  in  the  interest  of  a  reform  of  what  the  author  believes 
to  be  a  vicious  method  of  instruction,  the  following  remarks  may 
be  added  to  those  which  have  been  made  on  the  subject  in 
the  preceding  Preface.  In  the  first  place,  it  should  be  borne 
in  mind  that  the  introduction  of  an 'excessive  amount  of  mate- 
rial is  a  common  defect  in  modern  education,  and  that  the 
number  of  chemical  facts  involved  in  the  systematic  scheme  of 
analysis  is  as  great  as  can  be  properly  assimilated  by  the  stu- 
dent in  the  time  usually  devoted  to  the  course.  Secondly,  it 
is  to  be  considered  that  qualitative  analysis  is  a  satisfactory 
method  of  teaching  a  part  of  descriptive  chemistry  chiefly  be- 
cause it  unites  into  a  connected  whole  a  great  variety  of  isolated 
facts,  and  because  it  makes  evident  to  the  student  a  practical  use 
of  the  information  presented  to  him  ;  but  these  advantages  evi- 
dently do  not  apply  to  facts  not  directly  related  to  the  process 
of  analysis.  And  thirdly,  the  additional  knowledge  which  it  is 
most  desirable  that  the  general  student  of  descriptive  chemistry 
should  acquire  when  time  permits,  is  not  a  more  extended  ac- 
quaintance with  metathetical  test-tube  reactions,  which  involve  for 
the  most  part  merely  questions  of  solubility,  but  rather  a  knowl- 
edge of  new  principles  and  new  methods  of  manipulation  —  infor- 
mation that  would  be  much  better  given  by  a  course  in  inorganic 

preparations. 

ARTHUR  A.  NOYES. 

Boston,  March,  1897. 


PART    I. 


DIRECTIONS    FOR   LABORATORY   WORK. 


SEPARATION   OF   THE   METALS    INTO    GROUPS. 


i.  As  an  example  of  the  method  of  separation  of  the  metals 
into  groups,  prepare  a  solution  of  Pb(NO3)2,  Cu(NO3)2,  Zn(NO3)«, 
Ca(NO3)2,  Mg(NO3)2,  and  KNO3,  by  mixing  together  in  a  beaker 
about  5  cc.  of  each  of  these  solutions.  Add  to  it  HC1  little  by 
little,  as  long  as  a  precipitate  continues  to  form  (2  or  3  cc.  at 
most) ;  filter,  and  wash  with  cold  water,  till  the  washings  (which 
should  be  added  to  the  filtrate)  are  no  longer  acid  to  litmus  paper. 
Heat  the  filtrate  to  boiling,  and  pass  H2S  into  it  until  it  smells 
strongly  of  the  gas,  after  removing  it  from  the  delivery  tube,  blow- 
ing away  the  gas  collected  on  the  surface,  and  shaking  the  liquid. 
Shake,  heating  at  the  same  time ;  allow  the  precipitate  to  settle 
and  filter ;  wash  with  hot  water  till  free  from  acid,  throwing  the 
washings  away.  Add  to  the  filtrate  10  cc.  of  NH4C1,  NH4OH 
till  slightly  alkaline,  and  colorless  (NH4)2S,  until  the  solution  after 
shaking  blackens  a  piece  of  filter  paper  moistened  with  lead  ace- 
tate held  above  it.  Heat  and  shake  the  solution  till  the  precip- 
itate settles  quickly ;  filter,  and  wash  with  hot  water  containing  a 
little  (NH4)2S.  Add  (NH4)2CO3  to  the  filtrate  as  long  as  a  pre- 
cipitate forms ;  filter,  and  wash  the  precipitate.  To  one  portion 
of  the  filtrate  add  Na2HPO*.  Evaporate  the  remainder  of  the 
filtrate  nearly  to  dryness,  and  try  the  color  imparted  to  the  flame, 
using  a  platinum  wire. 

State  in  the  note-book  in  the  case  of  this  and  all  subsequent 
experiments  the  reason  of  each  operation  and  the  purpose  of  the 


8  DIRECTIONS  FOR  LABORATORY  WORK. 

addition  of  each  reagent.  Write  every  reaction  occurring  in  the 
process.  Name  the  other  metals  which,  if  present,  would  be 
precipitated  in  this  experiment  by  each  of  the  group  reagents. 
See  page  15.  Read  the  General  Directions,  pages  13-14. 

2.  As   an   example  of  the  separation   of   the    copper  and  tin 
groups,  pass  H2S  into  a  hot  solution  Of  HgCl2  and  AsCl3  until  the 
liquid  is  saturated.     Filter,  and  wash  the  precipitate ;  warm  some 
of  it  in  a  porcelain  dish  with  10  or  12  drops  of  (yellow)  (NH4)2SX 
diluted  with  a  little  water;  filter,  and  add  HC1  to  the  filtrate  till 
slightly  acid.     For  the  sake  of  comparison,  add  HC1  to  10  or  12 
drops  of  pure  (NH4)2SX  diluted  with  water.     (Read  the  notes  on 
page  19.) 

COPPER    GROUP. 

3.  Prepare  a  solution  of  Pb(NO3)2,  AgNO3,  and  HgNO3,  using 
10  cc.  of  each  solution.     Proceed  with  the  separation  as  directed 
on  pages  16  and  17. 

4.  In  order  to  become  familiar  with  the  color  and  appearance  of 
the  various  sulphides,  add  H2S  water  to  solutions  of  PbCl2,  HgCl2, 
BiCl3,  CdCl2)  Cu(NO3)2,  each   taken   separately,   and   previously 
acidified  with  a  few  drops  of  HC1.     In  the  case  of  HgClo,  try  the 
effect  of  adding  a  small  quantity  of  H2S  at  first,  then  of  adding  an 
excess,  shaking  in  each  case. 

5.  Prepare  a  mixed  solution    of  these   five   substances;  dilute 
with  water,  and  proceed  according  to  pages  17-22,  omitting  the 
treatment  with  (NH4)2SX  described  on  page  19. 

TIN  GROUP. 

6.  Pass   H2S  into  separate  solutions  ot   As?Cl8,    SbCl3,   SnCl2, 
and    SnCl4 ;   also  into  a   cold    and  then  into    a   hot   solution   oi 
Na3AsO4  acidified  with  HC1.     Note  the  color  and  appearance  of 
the  precipitates. 

7.  Prepare  a  mixed  solution  of  AsCl3,  SbCl*,  and  SnCl2,  using 
10  cc.  of  each.     Dilute  with  100  cc.  of  water  ;  heat  to  boiling,  and 
pass  in  H2S  till  saturated.     Filter,  and  proceed  with  the-  precip- 
itate according  to  pages  24  and  25. 


DIRECTIONS  FOR  LABORATORY   WORK:. 


ALUMINUM    AND    IRON    GROUPS. 

8.  Ferrous  and  Ferric  Salts.  —  Add   KCNS,    K3Fe(CN)6    and 
K4Fe(CN)6  to  separate  portions  of  FeSO4  solution. 

9.  Add  the  same  reagents  to  separate  portions  of  FeCl3  solu- 
tion.    Contrast  the  results  obtained  in  the  two  cases. 

10.  Try  the  following  experiments  illustrating  methods  of  oxi- 
dizing ferrous  to  ferric  salts,  and  read  in  this  connection  pages 
27-29. 

a.  Boil  2  to  3  cc.  of  FeSO4  solution  with  a  small  quantity  of 
HN03. 

b.  Boil  2  to  3  cc.  of  FeSO4  solution  with  an  excess  of  bromine 
water. 

c.  Add  HC1  and  a  few  crystals  of  KC1O3  to  a  little  FeSG4  solu- 
tion, and  heat. 

In  each  of  these  three  cases,  dilute  a  portion  of  the  solution 
thus  obtained  with  water,  and  test  it  with  K3Fe(CN)6  to  see 
whether  the  iron  has  been  completely  oxidized.  If  this  is  found 
not  to  be  the  case,  add  more  of  the  oxidizing  agent  to  the 
remainder  of  the  solution,  and  boil  again. 

11.  Try  the  following  experiments  illustrating  the  reduction  of 
ferric  salts : 

a.  Pass  H2S  into  a  solution  of  FeCl3. 

b.  Add  HC1  and  a  piece  of  zinc  to  a  little  FeCl3  solution  in  a 
test  tube. 

c.  Add  H2SO3  to  a  little  FeCl3  solution,  and  heat. 

In  each  case  continue  the  action  till  the  solution  is  decolorized, 
and  then  test  a  portion  with  KCNS  to  show  that  complete  re- 
duction has  taken  place. 

12.  Behavior  towards  General  Reagents.  —  To  solutions  of  alum, 
chrome    alum,    FeSO4,    FeCl3,   Zn(NO3)2,    MnSO4,   NiSO4,   and 
Co(NO3)2,    each   taken    separately,    add   NH4OH,   first   in    small 
quantity,  and  then  in  moderate  excess.    To  solutions  of  FeSO4  and 
MnSO4  add  an  equal  volume  of  NH4C1,  and  then  add  NH4OH. 

13.  Repeat  the  experiments,  using  NaOH  in  place  of  NH4OH. 

14.  Add  colorless  (NH4)2S  in  small  quantity  to  solutions  of  the 
'•iame  substances. 

15.  Separation  in  the  Absence  of  Phosphates.  —  Prepare  a  mixed 
solution  of  alum,  FeSO4,  Zn(NO3)2,  MnSO4,  NiSO4,  and  Co(NO,)2l 
"nd  analyze  it  as  directed  on  pages  31-33  and  36-38. 


to  DIRECTIONS  FOR  LABORATORY   WORK. 

16.  Experiments  Illustrating  the  Behavior  of  Phosphates.  —  Dis- 
solve some    CaCO3  in   dilute  HC1,  boil  a  few  minutes   to  expel 
the  CO.J,  and  add  NH4OH  till  alkaline.     Note  that  no  precipitate 
forms;  then  add  (NH4)2CO3. 

17.  'Dissolve  some  Ca3(PO4)2  in  dilute  HC1,  add  NH4OH  till 
alkaline ;  filter,  and  add  (NH4)2CO3  to  the  filtrate. 

1 8.  Dissolve  another  portion  of  Ca3XPO4)2  in  dilute  HC1,  add 
10  to  15  cc.  of  FeCl3  solution,  add  NH4OH  till  alkaline;  filter, 
and   add  (NH4)2CO3  to  the  filtrate. 

Explain  fully  the  significance  of  these  three  experiments  in  the 
note-book.     Read  carefully  pages  34  and  35  in  this  connection. 

19.  Separation  in  the  Presence  of  Phosphates.  —  A  solution  con- 
taining the  phosphates  of  barium,  strontium,  calcium,  and  magne- 
sium, and  salts  of  iron,  chromium,  aluminum,  manganese,  and  zinc, 
will  be  found  prepared  in  the  laboratory.     (Nickel  and  cobalt  are 
omitted,  since  their  separation  is  exactly  the  same  as  in  the  absence 
of  phosphates.)     Take  50  cc.  of  this  solution,  and  proceed  with  it 
according  to  pages  29-30  and  40-43,  omitting,  however,  the  anal- 
ysis of  the  alkaline-earth  sulphates. 

ALKALINE-EARTH    GROUP. 

20.  Prepare  a  mixed  solution  of  BaCl2,  SrCl2,  Ca(NOs)2,  and 
Mg(NO3)2,  and  proceed  with  it  according  to  pages  44-47. 

ALKALI    GROUP. 

21.  Add  H2PtCl6    to    separate    solutions  of    KC1,  NaCl,  and 
NH4C1. 

22.  Warm  a  little  solid  NH4C1  with  CaO  moistened  with  water  ;. 
note  the  odor  of  the  gas,  and  its  action  on  red  litmus  paper. 

ACIDS. 

23.  General  Tests.  —  Add  BaCl2  to  a  solution  of  Na2HPO4,  and 
to  one  of  Na2SO4.     Then  add  HC1.     State  in  the  note-book  the 
other  acids  which,  if  present,  react  in  the  same  manner  as  either 
of  these.     See  pages  51  and  52. 

24.  Add  AgNO3    to    separate   solutions   of    K2CrO4,    Na2SO4r 
Na2HPO4,  Na2B4O7,  (NH4)2C2O4,  Na,CO3,  NaCl,  KBr,  KI,  KCN, 
and  Na2S.     Note  the  color  of  each  precipitate.     Then  add  HNOs 
to  each  in  moderate  quantity.     See  page  53. 


DIRECTIONS    FOR    LABORATORY    WORK.  u 

25.  Special  Tests.  —  Try  with  the  solid  salts  and  solutions  pre- 
pared for  the  purpose  in  the  laboratory  all  the  special  tests  for 
acids  described  on  pages  55-64.       Make  the  reduction  tests  for 
chromates  with  a  small  quantity  of  K2CrO4  solution,  (i)  by  adding 
HC1  and  a  lump  of  zinc,  (2)  by  adding  considerable  strong  HC1 
and  boiling,  (3)  by  acidifying  with  HC1  and  passing  in  H2S. 

DRY    REACTIONS. 

26.  Heat  in  a  closed  tube,  as  described  on  page  66,  each  of 
the  following  substances:  (i)  Ni(NO3)2.6H2O  ;    (2)  KHC4H4O6; 
(3)  HgO;  (4)  NH4C1;  (5)  FeS2 ;  (6)  KC1O3 ;  (7)  ZnSO4. 

27.  Try  the  test  with  concentrated  H2SO4,  described  on  page 
70,    with   the   following   substances:    (i)    KHC4H4O6;   (2)    KI  ; 
(3)  KBr;  (4)  NH4C1;  (5)  Na2S2O3 ;   (6)  KNO3 ;  (7)  NaC2H3O2. 

28.  Heat  on  charcoal,  as  described  on  page  68,  the  following 
substances  :  (i)  KC1O3 ;  (2)  ZnO  ;  (3)  BiOCl ;  (4)  PbS  ;  (5)  As2O3, 
(using  a  very  small  quantity)  ;  (6)  CuSO4. 

29.  Try  the   NaPO3  bead  test   described  on  page   69,   with  : 

(1)  Mn02 ;-  (2)FeS04 ;  (3)  Cr2(SO4)3 ;    (4)  (CuSO4)  ;  (5)  feldspar. 

30.  Try  the  flame  test  described  on  page  70,  with  :  (i)  Na2SO4 ; 

(2)  K2SO4V  (3)  CaSO4 ;  (4)  SrSO4 ;  (5)  BaSO4 ;  (6)  CuSO4.     Omit 
the  moistening  with  H2SO4. 

31.  Examine  with  the  spectroscope  the  salts  provided  for  the 
purpose.     Record  the  position  on  the  scale  of  the  characteristic 
lines  of  each  element.      Then  analyze  spectroscopically  the  un- 
known mixtures.     See  page  71. 

UNKNOWN    SUBSTANCES. 

32.  After  completing  the  above  preliminary  work,  ask  for  un- 
known substances  for  analysis.     Test  the  solubility  of  the  first  five 
(or  ten)  substances  in  water  and  dilute  HC1,  and  in  addition  apply 
to  them  only  the  dry  tests  described  on  pages  65-70,  and  report  as 
far  as  possible  the  nature  of  their  constituents.     Make  complete 
analyses  of  the  remaining  substances  as  follows  :     Note  first  the 
physical  properties,  make  the  closed  tube  test  described  on  page  66, 
get  the  substance  into  solution  (pages  72-80),  and  analyze  the  solu- 
tion for  metals  according  to  pages  16-49.     Prepare  a  solution  for 
the  analysis  for  acids  (pages  81-83),  and  test  for  these  according 
to  pages  52-64.     Describe  the  behavior  of  the  substance  through- 


12  DIRECTIONS  FOR   LABORATORY   WORK. 

out  the  course  of  the  analysis  in  the  note-book.  In  reporting  the 
results  of  the  analysis  of  complex  substances,  do  not  simply  enu- 
merate the  metals  and  acids  found,  but  indicate  the  relative  quan- 
tities of  the  various  components,  and  state,  if  possible,  the  name 
of  the  substance.  Compare  its  properties  with  the  description  of 
it  given  in  the  larger  text-books  in  the  library. 


PART   II. 


THE    PROCESS    OF    ANALYSIS. 


GENERAL    DIRECTIONS. 


THE  first  aim  of  the  student  in  analytical  work  should  be  to 
secure  absolute  certainty  as  to  the  correctness  of  his  results ;  the 
next,  to  carry  out  the  processes  of  analysis  with  rapidity.  To 
attain  these  ends,  the  analyst  must  not  only  thoroughly  understand 
in  all  their  bearings  the  various  operations  which  he  performs,  but 
he  must  also  possess  skill  in  manipulation.  Thoughtless  or  unin- 
telligent following  of  directions  on  the  one  hand,  and  lack  of  neat- 
ness or  thoroughness  in  the  performance  of  the  operations  on  the 
other,  are  equally  sources  of  error.  The  student  will  acquire  the 
necessary  knowledge  by  practice,  if  accompanied  by  a  careful 
study  of  the  principles  of  the  process  and  by  the  habit  of  asking 
himself  the  purpose  of  each  operation.  The  details  of  manipula- 
tion will  be  learned  from  practice  and  instruction  in  the  labora- 
tory. The  following  general  directions,  which  apply  throughout 
the  whole  scheme  of  analysis,  should  be  always  observed  : 

Precipitation.  —  In  precipitating  a  solution  always  add  the  re- 
agent gradually  and  only  until  a  further  addition  produces  no  fur- 
ther precipitate.  This  can  be  determined  in  all  cases  by  allowing 
the  precipitate  to  settle,  or  by  filtering  a  little  of  the  liquid,  and 
adding  to  the  clear  solution  another  drop  of  the  reagent.  In  many 
cases,  for  example  in  precipitating  with  hydrogen  sulphide,  ammo- 


14  GENERAL    DIRECTIONS. 

nia,  or  ammonium  sulphide,  it  is  more  simply  determined  by  the 
odor  or  color  of  the  solution,  since  the  reagent  can  impart  its 
characteristic  properties  to  the  solution  only  when  an  excess  of  it 
has  been  added.  By  adding  the  reagent  in  this  way,  two  impor- 
tant results  are  attained :  first,  it  avoids  an  unnecessary  excess 
of  the  reagent,  by  which  the  precipitate  may  be  somewhat  dis- 
solved, the  solution  diluted,  and  subsequent  tests  made  less  deli- 
cate ;  and  second,  the  precipitation  is  thus  proved  to  be  complete. 
This  second  result  is  so  essential  that  any  uncertainty  in  regard  to 
it  is  entirely  inadmissible.  It  is  not  enough  for  the  analyst  to  be 
lieve  that  he  has  added  the  reagent  in  sufficient  quantity  to  pro- 
duce the  desired  result  —  he  must  conclusively  prove  that  such  is 
the  case  by  adding  more  of  it  to  the  filtrate. 

Washing  Precipitates.  —  Always  wash  a  precipitate  until  the  wash 
water  will  no  longer  give  a  test  for  ?.ny  substance  known  to  be 
present  in  the  filtrate.  The  object  of  precipitation  being  to  sepa- 
rate one  or  more  bodies  from  others  in  solution,  it  is  obvious  that 
this  result  is  only  incompletely  attained,  if  the  precipitate  is  not 
entirely  freed  from  the  liquid  in  which  it  was  thrown  down.  For 
example,  if  the  precipitate  produced  by  hydrogen  sulphide  is  not 
thoroughly  washed,  metals  of  the  succeeding  groups  will  be  pres- 
ent in  it,  and  will  cause  confusion  and  perhaps  error  by  their 
unexpected  appearance  in  the  course  of  the  analysis  of  the  pre- 
cipitate. Here,  again,  the  analyst  must  not  merely  guess  that  the 
precipitate  is  thoroughly  washed,  he  must  invariably  convince  him- 
self of  it  by  making  appropriate  tests.  If  the  filtrate  contains 
free  acid  or  alkali,  it  is  done  very  simply  by  testing  the  wash  water 
with  litmus  paper  ;  if  it  contains  chloride,  by  testing  with  silver 
nitrate,  etc.  On  complete  precipitation  by  the  various  reagents, 
and  on  proper  washing  of  the  precipitates,  the  success  of  an 
analysis  very  greatly  depends. 

Impurities  in  Reagents.  —  Bear  constantly  in  mind  that  the  sub- 
stances to  be  tested  for  may  be  present  in  the  reagents  as  im- 
purities. In  any  case  where  this  seems  possible,  make  sure  in 
regard  to  it  by  testing  the  reagent  directly.  The  habit  of  thus 
making  blank  tests  with  reagents  is  an  exceedingly  important  one 
to  acquire  ;  for  the  neglect  of  this  precaution  is  often  the  cause 
of  serious  errors. 


DETECTION    OF    THE    METALS. 


SEPARATION    OF    THE    METALS    INTO    GROjLJPS. 
Outline  of  the  process. 

Solution  containing  all  the  metals :  add  HCl. 


Precipitate  : 

Filtrate  :  add  IhS. 

AgCl,  HgCl, 

Precipitate  : 

Filtrate:  add  NH^O  Hand  (NH£<£. 

PbCl2. 

HgS,  PbS,  Bi2S3,  CdS, 

CuS,As2S3,  Sb2S8,SnS, 

Precipitate  : 

Filtrate  : 

SnS,.    Add  (NH^S^. 

A1O3H3, 

add  (NH^COz. 

CrO8H3, 

Residue  : 

Solution: 

CoS,  XiS, 

Precipitate  : 

Filtrate  ; 

HgS,  PbS, 

(NII4)3AsS4, 

FeS,  ZnS, 

BaC03, 

Mg,  K,  and 

Bi2S3,  CdS, 

(NH4)3SbS4, 

MnS. 

SrCO3, 

Na  salts. 

CuS. 

(NH  i)2SnS8. 

CaC03. 

Definition  of  the  groups  according  to  Freseniits. 

GROUP  I.  Alkali  Group.  —  Metals  not  precipitated  by  any  of 
\  le  general  reagents:  K,  Na,  NH4. 

GROUP  II.  Alkaline-earth  Group.  —  Metals  not  precipitated  by 
sulphides  either  in  acid  or  alkaline  solution,  but  precipitated 
by  carbonates :  Ba,  Sr,  Ca,  Mg. 

GROUP  III.  Aluminum  Group.  —  Metals  not  precipitated  by 
H2S  in  acid  solution,  but  precipitated  by  (NH4)eS  as  hydrates: 
Al,  Cr. 


i6 


DETECTION  OF   THE    METALS. 


GROUP  IV.  Iron  Group.  —  Metals  not  precipitated  by  H2S  in 
acid  solution,  but  precipitated  by  (NH4)2S  as  sulphides  :  Fe,  Co, 
Ni,  Mn,  Zn. 

GROUP  V.  Copper  Group.  —  Metals  precipitated  by  H2S  in  acid 
solution,  whose  sulphides  are  insoluble  in  (NH4)2SX:  Ag,  Pb,  Hg, 
Bi,  Cu,  Cd. 

GROUP  VI.  Tin  Group.  —  Metals  precipitated  by  H2S  in  acid 
solution,  whose  sulphides  are  soluble  in  (NH4)2SX:  As,  Sb, 
Sn,  (Au,  Pt). 

PRECIPITATION    AND    SEPARATION    OF    LEAD,    SILVER,    AND 
MERCUROUS    MERCURY. 

Outline  of  the  process. 


Precipitate :    AgCl,  HgCl,  PbCl2.     Add  hot  water. 


Residue  :  AgCl,  HgCl.     Add  NH±OH. 

-••    \"« 

Solution 

Add  H-iSO^ 
to  one  part. 

:  PbCl2. 

Add  H^S 

to  another. 

Residue  : 
NH2Hg  Cl 
+  Hg, 

Solution:  add  HNO*. 

Precipitate:    AgCl. 

Precipitate  : 
PbSO4. 

Precipitate  : 
PbS. 

Procedure.  —  Add  to  the  cold  solution  dilute  HC1  little  by 
little  as  long  as  the  precipitate  continues  to  increase ;  let  it 
stand  a  few  minutes ;  filter,  and  wash  with  a  small  quantity 
of  cold  water,  adding  the  washings  to  the  nitrate. 

Notes. —  i.  Non-formation  of  a  precipitate  proves  absence 
of  silver  and  mercurous  mercury,  but  not  of  lead,  as  PbCl2 
is  somewhat  soluble  in  water.  The  more  HC1  added,  the 
more  complete  will  be  the  precipitation  of  the  lead;  for  it, 
like  most  chlorides,  is  less  soluble  in  dilute  HC1  than  in 
water. 

2.  A  great  excess  is  to  be  avoided,  as  it  interferes  with 
the  H2S  precipitation  of  the  copper  and  tin  groups. 


PRECIPITATION   WITH  HYDROGEN  SULPHIDE.  17 

3.  Moreover,  strong  HC1  dissolves  all  three  of  the  chlo- 
rides quite  readily. 

4.  If  the  solution  is  concentrated  and  strong  HC1  is  added, 
certain  chlorides  soluble  in  water  but  less  soluble  in  HC1  may 
precipitate  ;  for  example,  BaCl2,  NaCl,  etc. 

5.  Dilute  HC1  may  also  precipitate  BiOCl  or  SbOCl,  but 
these  dissolve  readily  on  the  addition  of  more  HC1. 

6.  HC1  may  precipitate  from  a  solution  originally  alkaline 
many  other  substances,  namely,  any  substance  held  in  solu- 
tion by  an  alkaline  solvent;  for  example,  As2S3  from  (NH4)2S 
solution  ;  AgCN  or  Ni(CN)2  from  KCN  solution  ;  silicic  acid 
from  sodium  silicate ;   or  metallic  hydrates  from  solution  in 
caustic  alkalies.     Such  precipitates,  if  not  dissolved  by  more 
HC1,  must  be  treated  according  to  the  process  used  in  pre- 
paring the  solution  of  a  substance  for  analysis. 

Procedure.  —  Pour  boiling  water  through  the  filter  and 
test  one  portion  of  the  tiltrate  with  H2SO4,  and  another 
with  H2S.  If  lead  is  present,  wash  the  precipitate  with 
hot  water  till  the  wash  water  no  longer  reacts  for  lead. 
Then  pour  NH4OH  through  the  filter,  and  acidify  the  filtrate 
with  HNO3. 

Notes.  —  i.  If  the  PbCl2  is  not  completely  washed  out,  it 
changes  on  addition  of  NH4OH  to  a  white  basic  salt,  which 
passes  through  the  filter  and  gives  a  turbid  filtrate,  which 
becomes  clear,  however,  when  HNO3  is  added. 

2.  AgCl  unites  with  NH4OH   to  form   a  soluble  double 
compound,  which  is  destroyed  by  HNO3  with  reprecipitation 
of  AgCl. 

3.  The  black  residue  on  the  filter  is  probably  a  mixture  of 
NH2HgCl  and  finely  divided  Hg: 

2HgCl  +  2NH4OH  =  NH2HgCl  +  Hg  +  NH4C1  +  2H2O. 

PRECIPITATION    OF    THE    COPPER    AND    TIN    GROUPS. 

Procedure.  —  Pass  H2S  into  the  hot,  slightly  acid  solution, 
until  the  odor  of  the  gas  is  distinctly  perceptible  after  shak- 
ing. Heat  to  boiling,  allow  the  precipitate  to  settle,  filter, 
and  wash  the  precipitate  with  hot  water  till  free  from  acid. 


1 8  DETECTION  OF    THE    METALS. 

Dilute  a  small  portion  of  the  filtrate  with  two  or  three  times 
its  volume  of  water,  and  pass  in  more  H2S. 

Notes.  —  i.  In  addition  to  the  sulphides  of  these  groups  a 
white  precipitate  of  free  sulphur  is  formed,  if  the  solution  con- 
tains any  strong  oxidizing  agent ;  for  example,  nitric  acid  or 
a  nitrate,  a  chlorate,  a  chromate,  or  a  ferric  salt.  If  a  chro- 
mate  is  present,  the  solution  changes  in  color  from  red  to 
green ;  if  a  ferric  salt,  the  solution  becomes  colorless. 

2.  It  is  useless  to  pass   H2S  into  a  solution   containing 
much  HNO3  or  aqua  regia.     If,  therefore,  these  acids  have 
been  used  in  dissolving  the  substance,  the  solution  should 
be  evaporated  almost  to  dryness  and  the  residue  dissolved  in 
water  and  a  little  HC1,  before  passing  in  H2S. 

3.  The  solution  must  be  slightly  acid  to  prevent  the  pre- 
cipitation of  Zn,  Co,  and  Ni ;  but  too  much  acid  prevents  tfte 
precipitation  of  the  metals  of  the  copper  and  tin  groups,  espe- 
cially of  cadmium,  lead,  antimony,  and  tin.     One  part  of  acid 
(1.12  sp.  gr.)  to  20  or  25  of  water  is  a  suitable  concentration. 
If  much  more  dilute  than  this,  some  zinc  will  be  precipitated. 

4.  Complete  precipitation  may  fail  from  two  causes :  use  of 
an  insufficient  quantity  of  H2S  or  presence  of  too  much  HC1. 
Therefore  always  test  the  filtrate  by  passing  more  H2S  and  by 
further  dilution. 

5.  Dilution   with   water   before    passing    H2S    may    cause 
precipitation  of  BiOCl  or   SbOCl.      It  is  not  necessary  nor 
advisable  to  redissolve  these  precipitates  by  addition  of  more 
HC1 ;  for  H2S  changes  them  readily  to  sulphides. 

6.  Arsenic,  in  the  higher  state  of  oxidation,  is  precipitated 
by  H2S  very  slowly  indeed  in  the  cold,  and  not  very  rapidly, 
hot.     The  precipitate  is  As2S5 :  2H3AsO4  -f-  5H2S  =  As2S5  + 
8H2O.     The  slow  formation  of  a  yellow  precipitate  is  there- 
fore a  strong  indication  of  arsenic,  and  when  it  occurs  the 
solution  should  be  kept  nearly  boiling  during  precipitation. 

7.  Colors   of  the  Sulphides.  —  Black   or  brownish   black  : 
Ag2S,    HgS,    PbS,    Bi2S3,    CuS,    SnS.     Yellow:   CdS,   As2S3, 
As2S5,  SnS2.     Orange  :  Sb2S3,  Sb2S5.     Mercuric  salts  give  at 
first  a  white    precipitate    consisting   of   a  double    compound 
(HgCl2.2HgS),  which  turns  yellow,  red,  and  finally  black  with 
more  HaS,  owing  to  conversion  to  HgS.     Lead  salts  give  a 


SEPARATION  OF  COPPER   AND    TIN  GROUPS.  19 

red  precipitate  of  analogous   composition  when  precipitated 
by  H2S  from  a  solution  containing  considerable  HC1. 


SEPARATION    OF    THE    COPPER    AND    TIN    GROUPS. 

Procedure.  —  Warm  a  portion  of  the  H2S  precipitate,  or 
tne  whole  of  it,  if  small  in  amount,  with  ten  or  twelve  drops 
of  yellow  (NH4)2SX  diluted  with  a  little  water.  Filter,  and 
add  HC1  to  the  filtrate  till  it  reacts  acid  with  litmus  paper 
If  the  precipitate  which  forms  is  colored  yellowish  or  orange, 
treat  the  remainder  of  the  H2S  precipitate  in  the  same  way, 
using  rather  more  (NH4)2SX,  and  repeating  the  treatment 
with  successive  portions  of  it  till  the  filtrate  from  the  last 
treatment  is  precipitated  white  by  HC1.  Wash  the  residue 
insoluble  in  (NH4)2SX  with  water  till  the  wash  water  no  longer 
reacts  alkaline.  Collect  on  a  filter  the  precipitate  obtained 
by  the  addition  of  HC1  to  the  (NH4)2SX  solution,  wash  it 
#nce  or  twice  with  water,  and  dry  it  as  much  as  possible  by 
s  action. 

Notes.  —  i.  If  the  H2S  precipitate  has  not  been  thoroughly 
washed,  the  (NH4)2SX  will  precipitate  metals  of  the  aluminum 
And  iron  groups  contained  in  the  solution  adhering  to  it,  and 
these  will  appear  in  the  separation  of  the  copper  group. 

2.  Yellow  ammonium  sulphide  must  be  used,  for  the  color- 
less closs  not  dissolve  SnS.     A  pure  white  precipitate   after 
neutralization  with  HC1  is  taken  as  proof  of  the  absence  of 
As,  Sb,  and  Sn,  hence  the  use  of  very  little  (NH4)2SX  at  first; 
for,  otherwise,  the  large  amount  of  separated  S  would  conceal 
the  color  of  small  quantities  of  As2S5,  Sb2S5,  and  SnS2. 

3.  The  sulphides  dissolve  in  (NH4)2SX  with  formation  of 
salts  of  sulpho-acids : 

As?S8  +  3(NH4)2SX  =  2(NH4)3AsS4  +  (3*  —  5)S. 
SbaSs  +  3(MH4)2SX  =  2(NH4)3SbS4  +  (8*  —  5)S. 
SnS  -f  (NH4)2SX  =  (NH4)2SnS3  +  (*  —  2)S. 

4.  When  KC1  is  added  to  these    compounds,  the   higha 
sulphides  are  precipitated,  for  example  : 

(NH4),Sr,S8  +  WOl  *=  SnS2  +  2NH4C1  +  H2S. 


20 


DETECTION  OF   THE   METALS. 


5.  CuS  dissolves  somewhat  in  (NH4)2SX,  but  much  less  in 
Na2Sx.     HgS  dissolves  readily  in  Na2Sx,  but  not  in  (NH4)2SX. 
Hence,  when  the  preliminary  examination  or  the  color  of  the 
solution  indicates  Cu,  and  there  is  no  reason  to  suspect  Hg, 
use  Na2Sx. 

6.  When    Cu   is   present,  and  (NH4)2SX  is  used,  a  liver- 
colored  precipitate  is  obtained  on  neutralizing  with  HC1. 

SEPARATION    OF    MERCURY,    LEAD,    BISMUTH, 
CADMIUM,    AND    COPPER. 

Outline  of  the  process. 
Precipitate :  HgS,  PbS,  Bi2S3,  CdS,  CuS.     Boil  with  HNO$. 


Residue:  HgS. 
Dissolve  in  HCl 
and  KCIOz  ;  add 
SnCl*. 

Solution:  add  H^SO^ 

Precipitate: 
PbS04. 

Filtrate:  add  NH±OH. 

Precipitate  : 
Bi03H3. 
Dissolve  in  HCl, 
and  add  to  H^O. 

Filtrate  :  add  KCNandH^S. 

Precipitate:  HgCl. 

Precipitate: 
CdS. 

Solution: 
KCN.CuCN. 

Precipitate  : 
B50C1. 

Procedure. — Boil  the  residue  from  the  (NH4)2SX  treatment 
in  a  porcelain  dish  with  dilute  HNO3  (i  part  of  acid  1.2  spec, 
grav.,  and  3  of  water)  for  ten  minutes,  or  until  dissolved, 
replacing  the  acid  which  evaporates.  Filter  and  wash. 

Notes.  —  i.  HgS  remains  undissolved;  the  other  sulphides 
dissolve.  The  reactions  are  :  3PbS  +  8HNO3  =  3Pb(NO3)2 
+  3S  +  2NO  +  4H2O,  etc. 

2.  A  black  residue  is  not  necessarily  HgS,  but  may  be  S 
with  PbS,  CuS,  or  Bi2S3  inclosed  in  it.  The  confirmatory  test 
for  Hg  should  therefore  always  be  tried. 


COPPER   GROUP.  21 

3.  Continued   action   of   strong   HNO8  converts  HgS   to 
Hg(NO3).22HgS,  a  white  insoluble  compound.     A  heavy  white 
or  yellow  residue  should  therefore  be  tested  for  Hg. 

4.  Dilute   HNO3  oxidizes  PbS  partly,  and  strong  HNO8 
oxidizes   it   almost   entirely  to    PbSO4  :    3PbS  +  8HNO3  == 
3PbSO4  +  8NO  +  4H,O.     The  insoluble  residue  may  there- 
fore contain  PbSO4  as  well  as  HgS  ;  but  owing  to  the  solubil- 
ity of  PbSO4  in  HNO3,  some  Pb  will  always  pass  into  the 
filtrate. 

5.  The  insoluble  residue  may  also  contain  oxides  of  tin  and 
antimony,  especially  the  former,  if  the  (NH4)2SX  treatment  has 
been  insufficient. 


Procedure.  —  To  confirm  the  presence  of  Hg,  dissolve  the 
residue  by  heating  with  HC1  and  a  small  crystal  of  KC1O3. 
Boil  to  expel  the  excess  of  chlorine,  and  add  a  little  SnCl2 
solution. 

Notes.  —  i.  If  the  excess  of  chlorine  is  not  expelled  before 
adding  SnCl2,  the  reduction  of  the  HgCl2  is  prevented. 

2.  The  reaction  between  KC1O3  and  HC1  is  approximately 
as  follows : 

2KC1O3       4HC1  =  2KC1       C12        2C1O2       2H2O. 


Procedure.  —  To  the  filtrate  from  the  HgS  add  about  5  cc. 
of  strong  H2SO4,  and  evaporate  in  a  porcelain  dish  till  the 
white  fumes  of  H2SO4  come  off  thickly.  After  cooling  add 
about  50  cc.  of  water  and  pour  into  a  beaker ;  note  whether 
the  solution  is  at  all  turbid,  and  if  so,  filter  it. 

Notes.  —  i.     PbSO4  is  slightly  soluble  in  HNO3  ;  hence  the 
need  of  expelling  it. 

2.  PbSO4  is  slightly  soluble  in  concentrated  H2SO4 ;  hence 
the  dilution  before  filtering. 

3.  PbSO4  is  less  soluble  in  dilute  H2SO4  than  in  H2O  ;  it 
is  therefore  advantageous  to  have  the  acid  present  in  slight 
excess. 


n\ 


22  DETECTION  OF   THE   METALS. 

4.  Ammonium  salts,  for  example  NH4NO3,  also  dissolve 
PbSO4;   hence    the    need    of    thoroughly  washing    out    the 
(NH4)2SX  before  dissolving  the  H2S  precipitate  in  HNO3. 

5.  A  very  minute  precipitate  of  PbSO4  not  otherwise  notice- 
able may  be  detected  by  giving  a  rotary  motion  to  the  beaker, 
thus  causing  the  precipitate  to  collect  in  the  center. 

Procedure.  — To  the  filtrate  from  the  PbSO4  add  NH4OH 
till  alkaline,  and  then,  if  a  precipitate  forms,  a  small  quantity 
more,  in  order  to  redissolve  CdO2H2  and  CuO2H2.  Heat 
gently,  filter,  and  wash  the  precipitate.  Dissolve  it  by  pour- 
ing a  little  strong  HC1  through  the  filter;  evaporate  the 
solution  to  one  or  two  drops,  and  pour  into  a  beaker  of 
water. 

Notes. —  i.  If  the  H2S  precipitate  has  been  insufficiently 
washed,  A1O3H3,  FeO3H3,  etc.,  may  precipitate  on  the  addition 
of  NH4OH. 

2.  If  the  lead  has  not  been  completely  removed  by  close 
adherence  to  the  above  directions,  it  also  will  precipitate  at 
this  point. 

3.  The  formation  of  a  precipitate  by  NH4OH  must  there- 
fore never  be  taken  as  proof  of  the  presence  of  Bi,  but  the 
confirmatory  test  must  always  be  tried. 

4.  Reaction  :  BiCl3  +  H2O  —  BiOCl  -f  2HC1. 

5.  The  more  completely  the  HC1  is  removed  by  evapora- 
tion, the  more  delicate  will  be  the  test  for  Bi.     Time  is  some- 
times required  for  the  precipitation  of  the  BiOCl. 

6.  Sb   and  Sn  salts  like  those  of  Bi  are  precipitated  by 
H2O,  but  these  should  not  be  present  in  the  solution  at  this 
stage  of  the  analysis.     Tartaric  acid   prevents  the  precipita- 
tion of  Sb  salts  by  H2O,  but  not  that  of  Bi  or  Sn  salts. 

Procedure.  —  Unless  distinctly  blue,  test  a  small  portion  of 
the  filtrate  from  the  BiO3H3  for  Cu  by  acidifying  with  acetic 
acid  and  adding  K4Fe(CN)6.  If  the  ammoniacal  solution  is 
blue,  add  KCN  little  by  little  till  the  blue  color  due  to  the 
Cu  is  completely  discharged,  and  then  pass  in  H2S  for  a  few 
seconds. 


COPPF  f    GROUP.  23 

Motes.  —  i.     Cd  is  -.i^o  precipitated  by  K4Fe(CN)6 ;  but  the 
precipitate  is  white. 

2.  The  precipitation  with  K4Fe(CN)6  is  a  more  delicate 
^est  for  Cu  than  the  blue  coloration  with  NH4OH. 

3.  A  potassium  cuprous  cyanide  (KCN.CuCN)  is  formed 
oy  the  ac^on  of  the  KCN,  and  this  compound  is  not  decom- 
posed by  H2S.      Cd  forms  with  excess  of  KCN  the  double 
salt  2KCN.Cd(CN)2,  which  is  decomposed  by  H2S. 

4.  A  yellow  coloration    with  H2S    does  not   indicate  Cd. 
A  yellow  precipitate  must  be  obtained. 

5.  A  black  precipitate  (due   to  HgS,  PbS,  FeS,  etc.,)  is 
sometimes    obtained   in    testing   for  Cd  with  H2S,  owing  to 
some  previous  error  in  the  analysis.     To  determine  whether 
Cd  is  present  in  it,  filter  it  off,  wash  it,  and  boil  it  several 
minutes  with  an  excess  of  dilute  H2SO4  (one  part  of  acid  to 
four  of  water)  ;  filter,  dilute  the  filtrate,  and  saturate  it  with 
H2S.     H2SO4  of  this  strength  dissolves  CdS,  changes  PbS  to 
insoluble  PbSO4,  but  does  not  affect  CuS  or  HgS. 


ADDITIONAL  REACTIONS  OF  METALS  OF  THE 
COPPER  GROUP. 

1.  Behavior  towards  the  Alkalies.  — All  metals  of  this  group 
are   precipitated   both  by   NaOH  and   NH4OH.     Excess  of 
NaOH  dissolves  only  the  Pb  precipitate.     Excess  of  NH4OH 
dissolves  Ag2O,  CdO2H2,  and  CuO2H2.     The  precipitates  are 
hydroxides  in  the  case  of  Bi,  Cd,  and  Cu,  the  oxide  in  the 
case  of  Ag,  and  basic   salts   in    the  case    of  Pb.     Hg  gives 
with  NaOH,  Hg2O  (black)   and    HgO  (yellow);  while   with 
NH4OH  substituted  ammonium  compounds  are  formed,  e.g., 
NH2Hg2NO3  and  NH2HgCl. 

2.  Mcrcurous    arid    Mercuric    Salts.  —  These    are    distin- 
guished (i)  by  HC1,  which    precipitates  the  former  and  not 
the  latter;  (2)  by  NH4OH,  which  gives  a  black  precipitate 
with  the  former  and  a  white  one  with  the  latter.     The  reac- 
tions are : 

2HgNO8  +  2NH4OH  =  NH2Hg2NO3  +  NH4NO3  +  2H2O. 
HgCl2  +  2NH4OH  =  NH2HgCl  -f  NH4C1  +  2H,O. 


DETECTION  OF   THE   METALS. 


SEPARATION    OF    ARSENIC,    ANTIMONY,    AND    TIN. 
Outline  of  the  process. 

Precipitate:  As2S5,  Sb2S5,  SnS2.     Add  strong  HCl. 


Residue  :  ^8285. 
Dissolve  in  HCl  and 
KCIOz  ;  add  NH±OH, 
NH±Cl,  and  MgClz. 

Solution  :    SbCls,  SnCU,  (and  small  amount  of 
HsAsC^).    .Place  in  hydrogen  generator. 

Residue  :  Sn. 
Dissolve  in  strong 
HCl  and  add  HgCl* 

Gas  evolved  :  SbH3  (and  AsHg) 
Pass  through  a  hot  tube. 

Precipitate  : 
MgNH4AsO4. 

Deposit:    Sb  (and  As). 
Treat  with  NaOCL 

Precipitate  : 
HgCl. 

Residue  :               Solution  : 
Sb.                 (H3AsO4). 

Procedure.  —  Heat  gently  (not  to  boiling)  the  precipitated 
sulphides,  dried  as  much  as  possible  by  suction,  with  strong 
HCl  (1.2  spec,  grav.)  as  long  as  the  vapors  blacken  paper 
moistened  with  Pb(C2H3O2)2  and  NH4OH.  Dilute  with  a 
little  water,  filter,  and  wash  the  undissolved  residue.  Dis- 
solve it  in  a  little  HCl  with  addition  of  a  crystal  (or  two)  of 
KC1O3,  heat  to  expel  the  excess  of  chlorine,  add  NH4OH 
till  alkaline,  filter  if  turbid,  and  then  add  enough  more 
NH4OH  to  form  one  third  the  volume  of  the  solution  ;  add 
magnesia  mixture,  stir,  and  if  no  precipitate  forms  at  once, 
allow  it  to  stand  over  night. 

Notes.  —  i.  The  separation  of  As2S5,  Sb2S5,  and  SnS2  by 
HCl  is  not  perfect.  Some  Sb2S5  and  SnS2  may  remain  undis- 
solved, and  small  quantities  of  arsenic  pass  into  solution. 
It  is  to  prevent  the  latter  as  far  as  possible  that  the  acid  is 
only  warmed,  not  boiled. 

In  case  there  is  reason  to  believe  that  the  precipitate 


2. 


consists  mainly  of  As2S5,  warm  it  first  with  a  saturated  solu- 
tion of  (NH4)2CO3  (made  by  dissolving  the  finely  ground  solid 


TIN.    GROUP.  25 

salt  in  cold  water),  filter,  treat  the  precipitate  with  strong  HC1 
as  above  directed,  and  acidify  the  (NH4)2CO3  filtrate  with 
HC1.  (NH4)2CO3  dissolves  As,S5  without  difficulty,  but  dis- 
solves Sb2S5  and  SnS2  only  slightly. 

3.  If  a  yellow  turbidity  (Sb2S3  or  SnS2)  appears  on  diluting 
•  the  HC1  solution  with  v;ater,  it  shows  that  the  H2S  has  not 

been  completely  expelled. 

4.  On  the  addition  of  NH4OH  any  antimony  or  tin  not 
dissolved  out  of  the  mixed  sulphides  by  the  strong  HC1  will 
be  precipitated  as  hydrate. 

5.  MgNH4AsO4  like  MgNH4PO4  is  somewhat  soluble  in 
water,  but  much  less  so  in  NH4OH.      Hence   the   solution 
should   be   concentrated   and  an  excess  of  NH4OH   added. 
The  precipitate  is  crystalline,  and  adheres  to  the  sides  of  the 
vessel,  especially  along  the  lines  rubbed  with  the  glass  rod. 

6.  REACTIONS  : 

As2S5  +  10C1  +  8H2O  =  2H3AsO4  +  5S  +  10HC1. 
H3AsO4  +  3NH4OH  =  (NH4)3AsO4  +  3H2O, 
(NH4)3AsO4  +  MgCl2  =  MgNH4AsO4  +  2NH4C1. 

Procedure.  —  Place  a  piece  of  platinum  and  some  zinc  in 
a  flask  fitted  with  a  thistle  and  delivery  tube,  and  add  some 
dilute  HC1.  Cause  the  gas  to  pass  through  a  tube  contain- 
ing dry  CaCl2,  and  then  through  a  hard  glass  tube  drawn  to 
a  capillary  at  the  end  and  at  two  intermediate  points.  After 
the  air  has  been  expelled  (proved  by  collecting  some  of  the 
gas  in  a  small  test  tube  and  igniting  it),  light  the  gas  at  the 
end  of  the  apparatus,  and  heat  the  hard  glass  tube  just  back  \ 
of  the  first  capillary  with  a  small  Bunsen  flame.  Continue 
this  for  a  few  minutes  t )  see  that  no  mirror  forms ;  then 
pour  in  the  solution  of  SbCl3  and  SnCl4  little  by  little.  Note 
whether  any  deposit  forms  in  the  capillaj-y^  and  whether  the 
platinum  in  the  generator  becomes  blackened.  After  some 
minutes  pour  out  the  liquid  in  the  generator,  wash  the  resi- 
due by  decantation  with  water,  boil  with  strong  HC1  till  en- 
tirely dissolved,  dilute  the  solution,  and  add  HgCl2.  Break 
off  the  hard  glass  tube  just  beyond  the  mirror,  and  immerse 
it  in  a  test  tube  containing  NaOCl  solution.  Note  whether 
the  mirror  dissolves  wholly  or  in  part. 


DETECTION  OF    THE   METALS. 

Notes.  —  i.  SnCl4  is  reduced  by  nascent  hydrogen  to  metal- 
lic tin,  which  deposits  on  the  zinc.  A  part  only  of  the  Sb 
deposits  on  the  platinum,  the  remainder  being  further  reduced 
to  SbH3.  The  H3AsO4  is  reduced  to  AsH3  without  precipita- 
tion of  arsenic. 

2.  Heat   decomposes    SbH3  and  AsH3  into  antimony  or 
arsenic  and  hydrogen. 

3.  As  small  quantities  of  As2S5  are  dissolved   by  strong 
HC1,  a  deposit  in  the  capillary,  unless  accompanied  by  dis- 
tinct  blackening   of   the  platinum,  must   not   be    considered 
proof  of  the  presence  of  Sb.     The  test  with  NaOCl  removes 
all  doubt,  however ;  for  an  arsenic  mirror  dissolves  almost  in- 
stantly, forming  H3AsO4,  while  antimony  remains  unaffected 
for   a   long   time.     Even  when  the   deposit  consists  of  both 
metals,  it  is  easy  to  detect  the  presence  of  both ;  for  arsenic, 
being   more   volatile,    deposits   in   the   part   of  the  capillary 
more  remote  from  the  flame,  and  this  part  of  the  mirror  is 
seen  to  dissolve  completely  on  treatment  with  NaOCl.     This 
reagent  serves  therefore  not  only  to  confirm  the  presence  of 
antimony  but  also  to  detect  traces  of  arsenic  so  small  as  not 
to  be  shown  by  the  test  with  MgCl2  and  NH4C1. 

4.  The   presence   of   an    element    may  be   established  in 
three  ways  :  (i)  by  isolation  of  the  element  itself ;  (2)  by  for- 
mation of  one  of  its  characteristic  compounds  ;  (3)  by  causing 
a  change  to  take  place  in  some  other  substance.     The  test 
for  tin  with  HgCl2  is  one  of  the  few  examples   of   the  last 
method. 

ADDITIONAL    REACTIONS   OF   METALS   OF  THE 
TIN    GROUP. 

1.  Arsenic  and  antimony  may  be  detected  in  the  original 
solution,  even  while  other  metals  are  present,  by  the  generator 
test  above  described ;  and  where  minute  quantities  are  to  be 
detected,  that  test  should  be  employed,  since   the   color  of 
their  sulphides  may  be  concealed  by  the  sulphur  separated 
from  the  (NH4)2SX. 

2.  Arsenites  and  arseniates  are  distinguished  (i)  by  H2S, 
which  in  the  cold  precipitates  the  former  instantly,  and  the 
Matter  only  very  slowly;  (2)   by   AgNO3,  which   in  perfectly 


OXIDATION  AND  REDUCTION.  27 

neutral  solution  gives  a  yellow  precipitate  with  the  formert 
and  a  brown  one  with  the  latter ;  (3)  by  magnesia  mixture, 
which  precipitates  only  arsenic  acid. 

3.  Stannous  and  stannic   salts    are    distinguished   (i)  by 
HgCl2,  which  gives   a  precipitate   of  HgCl  or   Hg  with  the 
former,  and  none  with  the  latter;  (2)  by  H2S,  which  gives  a 
brownish-black  precipitate  with  the  former,  and  a  yellow  one 
with  the  latter. 

4.  Detection  of  Arsenic  in    Wall  Papers,  Fabrics,  etc.  —  Cut 
the  material   into  small   pieces  and  treat  it  as  described  in 
note  6,  £,  on  page  74. 

OXIDATION    AND    REDUCTION. 

As  many  of  the  metals  of  the  aluminum  and  iron  groups 
exist  in  two  or  more  different  states  of  oxidation,  a  few  gen- 
eral remarks  on  oxidation  and  reduction  may  be  made  with 
advantage  at  this  point. 

A  substance  is  said  to  be  oxidized  when  oxygen  or  some 
other  acid  element  or  radical  is  added  to  it,  or  when  hydrogen 
or  some  other  basic  element  or  radical  is  taken  from  it. 

A  substance  is  said  to  be  reduced  when  the  reverse  takes 
place.  • 

The  oxidation  of  one  substance  involves  the  simultaneous 
reduction  of  some  other  substance. 

In  the  following  examples  the  substance  oxidized  is  placed 
first: 

4FeO2H2  +  O2  +  2H2O  =  4FeO8Hs. 
3PbS  +  8HNO3  =  3PbSO4  +  8NO  +  4H2O. 
H3As03  +  C12  +  H20  =  H3As04  +  2HC1. 
SnCl2  +  2HgCl2  =  SnCl4  +  2HgCl. 
2K4Fe(CN)6  +  C12  =  2K3Fe(CN)6  +  2KC1. 

In  the  application  of  the  definition  of  oxidation  it  is  im- 
portant to  distinguish  between  the  addition  of  an  element 
and  the  substitution  of  one  element  or  radical  for  another. 
For  example,  PbO  dissolves  in  HNO3,  forming  Pb(NO3)2  and 
H2O.  This  is  not  an  oxidation,  but  a  simple  metathesis ;  for 
the  two  NO3  groups  are  not  added,  but  simply  take  the  place 
of  their  equivalent,  one  oxygen  atom.  But  in  the  reaction : 
3Pb  +  8HN03  =  3Pb(N08)2  +  2NO  +  4H2O,  the  lead  is 


28  DETECTION  OF   THE  METALS. 

oxidized,  the  NO3  groups  being  added  to  it,  and  the  HNOS 
is  reduced  to  NO.  The  following  additional  reactions  are 
given  to  illustrate  further  this  distinction.  The  first  reaction 
of  each  pair  is  an  oxidation  and  reduction;  the  second,  a 
metathetical  change : 

MnO2  +  4HC1  =  MnCl2  +  CJ2  +  2H3O. 

MnO  +  2HC1  =  MnCl2  +  H2O. 

SbCl3  +  3H  =  Sb  +  3HC1. 

SbCl3  +  H2O  —  SbOCl  +  2HC1. 

2K2CrO4  +  16HC1  =  2CrCl3  +  3C12  +  4KC1  •+-  8H2O. 

2K2CrO4  +  2HC1  =  K2Cr2O7  +  2KC1  +  H2O. 

In  order  to  write  reactions  involving  oxidation  and  reduc- 
tion, consider  first  the  oxides  corresponding  to  each  of  the 
substances  taking  part  in  the  reaction  and  known  to  be  pro- 
duced by  it.  It  will  then  be  evident,  on  the  one  hand,  how 
many  oxygen  atoms  the  oxidizing  agent  furnishes,  and  on  the 
other,  how  many  are  required  for  the  substance  oxidized,  thus 
showing  the  relative  number  of  molecules  of  each  entering 
into  the  reaction.  The  method  is  illustrated  by  the  following 
examples : 

Oxidation  of  FeSO4  with  HNO3.  — The  FeSO4  is  known  to 
be  oxidized  to  Fe2(SO4)3  and  the  HNO3  to  be  reduced  to  NO. 
Write  the  symbols  of  these  compounds  dualistically,  regard- 
ing them  as  composed  of  the  anhydride  of  the  acid  united 
with  water  or  the  oxide  of  the  metal.  FeSO4  =  FeO,  SO3 ; 
Fe2(S04)3  =  Fe203,  3SO3 ;  and  2HNO3  =  H2O,  N2O5.  Then 
2FeO  +  O  =  Fe2O3  and  N2O5  =  2NO  -f  O3.  That  is,  2HNO3 
furnish  3  atoms  of  oxygen,  and  i  atom  of  oxygen  oxidizes 
2  atoms  of  iron  from  the  ferrous  to  the  ferric  condition. 
Hence  2HNO3  oxidize  6FeSO4.  If  the  ferric  compound  thus 
formed  is  to  remain  in  solution,  free  acid  of  some  kind  must 
be  added.  Its  amount  is  readily  seen  by  inspection. 
The  reactions  are  the  following : 

6FeS04  +  2HN03  +  3H2SO4  =  3Fea(^O4).  +  2NO  + 
4H20. 

6FeSO4  +  2HNO:  +  6HNO3  =  2Fe2(SO4)3  +  2Fe(NO3), 
+  2NO  +  4H2O. 


ALUMINUM  AND  IRON  GROUPS.  29 

Reduction  of  K2Cr2O7   with    SnCl2.  —  SnCl2   corresponds 
to    SnO  ;    SnCl4  to  SnO2 ;    SnO2  =  SnO  +  O.     K2Cr2O7  = 
K2O,    2CrO3  ;     CrCl3     corresponds     to     Cr2O3  ;     2CrO3  = 
Cr2O3  +  O3. 
Therefore  3SnCl2  reduce  lK2Cr2O7,  and  the  reaction  is : 

K2Cr207  +  3SnCl2  +  14HC1  ==  2CrCl3  +  2KC1  +  SSnCU 
+  7H20. 

Oxidation  of  MnO2H2  by  KC1O3  in  presence  of  Na2CO,>  — 
KC1O3=:KC1  +  O3.  MnO2H2  =  MnO,  H2O.  Na,MnO4  = 
Na2O,  MnO3.  MnO3  =  MnO  -f-  O2. 

Hence  :  2KC1O3  +  3MnO2H2  +  3Na2CO3  =  2KC1  + 
3Na2MnO4  +  3CO2  +  3H2O. 

The  most  important  oxidizing  agents  from  an  analytical 
point  of  view  are  :  Cl,  Br,  and  J  solutions,  HNO3,  KC1O3, 
K2Cr2O7,  and  KMnO4. 

The  mos:  important  reducing  agents  in  solution  are  nascent 
hydrogen,  H2S,  H2SO3,  SnCl2  and  oxalic  acid  (H2C2O4) ;  at  a 
high  temperature,  C,  KCN,  starch  and  other  organic  bodies. 

It  is  well  to  remember  what  the  usual  reduction  products  of 
the  important  oxidizing  agents  are  :  Cl  and  Br  are  reduced  to 
HC1  and  HBr;  HNO3  is  reduced  to  NO;  KC1O3  to  KC1, 
K2Cr2O7  and  KMnO4  in  acid  solution  to  chromic  and  manga- 
nous  salts  respectively.  It  should  be  added,  however,  that 
the  reduction  product  of  an  oxidizing  agent  is  not  under  all 
circumstances  the  same.  For  example,  HNO3  is  reduced  by 
certain  metals  to  N2O  and  NH3;  ,KMnO4  in  neutral  solution 
is  reduced  only  to  MnO2,  not  to  MnO. 

ALUMINUM  AND  IRON  GROUPS. 

Preliminary   Tests. 

Procedure.  —  Boil  one  quarter  of  the  filtrate  from  the  H2S 
precipitate  until  the  excess  of  H2S  is  expelled.  Add  a  little 
HNO3  and  boil  again  to  oxidize  the  iron.  Divide  this  solu- 
tion into  two  unequal  portions.  Add  to  the  larger  portion 
10  cc.  of  NH4C1,  and  NH4OH  to  slight  but  distinct  alkaline 
reaction  ;  heat  to  boiling,  and  allow  to  stand  some  minutes  if 
no  precipitate  separates  at  once.  Note  whether  a  precipitate 


30  ^Hfc    DETECTION   OF   THE   METALS. 


forms  and  what  its  color  is.  Then  add,  without  filtering, 
colorless  (NH4)2S  in  small  quantity.  If  NH4OH  produces 
a  precipitate,  test  the  rest  of  the  solution  which  has  been 
boiled  with  HNO3  for  H3PO4  by  adding  an  equal  bulk  of 
(NH4)2  MoO4  solution,  stirring,  and  allowing  it  to  stand  some 
minutes. 


Notes. —  i.  If  NH4OH  produces  no  precipitate,  it  proves 
the  absence  of  iron,  chromium,  and  aluminum,  and  the  spe- 
cial tests  for  these  elements  may  then  be  omitted  in  the 
subsequent  analysis. 

2.  This    conclusion   is   reliable,  however,  only    when    the 
directions  are  exactly  followed ;  for  A1O3H3  dissolves  slightly 
in  excess  of  NH4OH,  CrO3H3  dissolves  in  the  cold  (giving 
a  pink  solution)  but  is  precipitated  on  boiling,  and  a  small 
precipitate  of  any  of  the  three  is  easily  overlooked,  owing  to 
its  transparency,  unless    time    is  allowed  for  the    precipitate 
to  gather  in  flocks. 

3.  Moreover,  non-volatile  organic  matter,  such  as  sugar, 
tartaric  acid,  etc.,  must  not  be  present,  for  it  prevents  entirely 
the  precipitation  of  these  three  elements  by  NH4OH. 

4.  Phosphates    of   alkaline-earth   metals,    if   present,    are 
also  precipitated  by  NH4OH.     The  advantage  in  testing  for 
H3PO4  at  this  point  will  be  made  clear  later,  and  the  cases 
in  which  it  is  useless  to  try  the  test  will  be  discussed. 

5.  The  color  of  the  NH4OH  precipitate  indicates  which  of 
the   metals  are  present.     A1O3H3  is  white  and  transparent  ; 
CrO3H3,  greenish  blue ;  and  FeO3H3,  reddish  brown. 

6.  The  color  of  the  sulphides  is  also  important :  FeS,  NiS, 
and  CoS  are  black;  ZnS  is  white;  and  MnS  is  flesh-colored, 
but  turns  brown  on  standing  in  the  air.     If  the  (NH4)2S  pre- 
cipitate is  pUre  white,  the  subsequent  tests  for  iron,  nickel, 
and  cobalt  may  be  omitted. 

7.  The  H2S  must  be  completely  expelled  at  the  beginning; 
otherwise,  (NH4)2S  is  formed,  and  the  NH4OH  test  is  of  no 
value. 

8.  In  whatever  condition  the  iron  was  originally  present,  it 
is  in  the  form  of  a  ferrous  salt  after  the  treatment  with  H2S  :  — 

2FeCl3  +  H2S  =  2FeCl2  +  2HC1  +  S.     The    solution   is 


ALUMINUM  AND  IRON  GROUPS.  ^        31 


boiled  with  HNO3,  since  ferrous  iron  is  not  precipitated  by 
NH4OH  in  the  presence  of  ammonium  salts. 

9.  A  black  precipitate  with  NH4OH  shows  incomplete  ex- 
pulsion of  the  H2S,  or  incomplete  oxidation  of  the  iron. 

10.  NH4C1  is  added  to  prevent  the  precipitation  of  MgO2H2 
and  MnOoH2,  which  otherwise  are  precipitated  by  NH4OH. 
Aside  from  this  it  is  useful,  for  it  greatly  promotes  the  sepa- 
ration of  A1O3H3.  and  of  all  the  sulphides. 

11.  If  manganese  is  present  the  solution  rapidly  becomes 
brown  after  the  addition  of  NH4OH,  owing  to  the  absorption 
of  oxygen  and  precipitation  of  MnO3H3. 

12.  (NH4)2S  changes  FeO3H3  to  FeS,  but  has  no  effect  on 
A1O3H3  and  CrO3H3.     A12S3,  Cr2S3,  and  Fe2S3  are  not  formed 
in  the  wet  way. 

13.  Even  if  present  originally  as  chromate,  the  chromium 
will  be  precipitated  by  NH4OH,  owing  to  reduction  of  the 
chromate  by  H2S. 

Precipitation  of  the  Aluminum  and  Iron  Groups. 

Procedure.  —  If  the  preliminary  tests  show  the  presence 
of  metals  of  the  aluminum  or  iron  groups,  place  the  rest  of 
the  filtrate  from  the  H0jS  precipitate  in  a  flask,  add  15  or 
20  cc.  of  NH4C1,  NH4OH  till  barely  alkaline,  and  (NH4)2S 
until  the  liquid  after  shaking  causes  blackening  of  a  piece 
of  Pb(C2H3O2)2  paper  held  above  it.  Heat  to  boiling,  and 
shake  until  the  precipitate  will  subside  readily.  Filter,  and 
wash  immediately  with  water  containing  a  very  little  (NH4)2S. 

Notes.  —  i.     The  notes  given  under  the  preliminary  tests  on 
the  preceding  page  apply  here  also. 

2.  Colorless  (NH4)2S  is  used  instead  of  yellow  (NH^S*, 
as   the  excess  of  'sulphur  is  troublesome  in  the   subsequent 
examination  of  the  nitrate.     For  the  same  reason  care  should 
be  taken  to  avoid  using  an  excess  of  (NH4)2S. 

3.  •  The  nitrate   from   the  (NH4)2S    precipitate  should  be 
colorless  or  light  yellow.     A  brown  or  black  color  indicates 
the  presence  of  nickel,  whose  sulphide  dissolves   somewhat 
in  excess  of  (NH4)2S,  from  which  it  should  be  precipitated  by 
acidifying  with  acetic  acid,  boiling,  and   filtering   through  a 


DETECTION  OF    THE    METALS. 


new  filter.  A  pink  color  indicates  chromium  dissolved  in 
excess  of  NH4OH,  from  which  solution  it  is  thrown  out  on 
boiling.  A  green  color  is  due  to  traces  of  finely  divided 
FeS  in  suspension,  which  separate  on  standing. 

4.  All  five  sulphides  oxidize  rapidly  in  the  air  to  sulphates  ; 
hence  the  addition  of  (NH4)2S  to  the  wash  water. 

5.  NH4OH  slowly  absorbs  C!O2  from  the  air;  hence  if  the 
solution  is  allowed  to  stand  for  some  time  before  filtering, 
BaCO8,  SrCO3,  and  CaCO3  may  precipitate. 

Separation  of  Nickel  and  Cobalt. 
Outline  of  the  process. 

Precipitate :  A1O3H3,  CrO3H3,  CoS,  NiS,  FeS,  MnS,  ZnS,  [Ba3(PO4)2, 
Sr3(PO4)2,  Ca3(PO4)2,  MgNH4PO4].      Treat  with  dihite  HCl. 


Residue  :  CoS,  NiS,  (FeS  in  small  amount).     Dissolve  in  aqua 
regia  and  add  Nff^OH. 


Precipitate  : 
Fe03H3. 


Filtrate  :  expel  NH±  salts,  add  KNO^  and 


Precipitate : 
Co(NO2)3.  3KNO2. 


Solution:  add NaOH. 


Precipitate:  NiO2H2. 
Test  in  borax  bead. 


Solution. 


Procedure.  —  Treat  the  (NH4)2S  precipitate  in  a  dish  with 
cold  dilute  HCl,  made  by  diluting  one  part  of  acid  (1.12  spec, 
grav.)  with  five  of  water ;  stir,  filter  when  dissolved  or  after 
standing  a  few  minutes,  and  if  a  black  residue  remains, 
wash  it  thoroughly.  Separate  it  from  the  filter,  or,  if  small 
in  amount,  incinerate  the  filter  in  a  porcelain  crucible,  and 
dissolve  the  residue  in  a  little  aqua  regia.  Dilute,  add 
NH4OH  in  moderate  excess,  and  filter.  Add  to  a  small 
portion  of  the  filtrate  a  drop  or  two  of  (NH4)2S.  If  a 
black  precipitate  is  produced,  evaporate  the  remainder  to 


ALUMINUM  AND  IRON  GROUPS.  33 

dryness,  ignite  till  the  ammonium  salts  are  expelled,  and 
test  a  little  of  the  residue  in  a  borax  bead  in  the  flame  of 
the  blowpipe.  Dissolve  the  residue  in  a  little  aqua  regia, 
evaporate  to  two  or  three  drops,  add  50  cc.  of  KNO2  solu- 
tion and  HC2H3O2  to  strongly  acid  reaction.  Allow  the 
mixture  to  stand  some  hours  in  a  warm  place,  filter,  add 
NaOH  to  the  filtrate  till  alkaline,  and  if  a  precipitate  forms, 
filter,  and  test  the  precipitate  in  a  borax  bead  in  the  oxidiz- 
ing flame. 

Notes. —  i.  The  acid  used  is  cold  and  dilute  to  avoid  dis- 
solving NiS  and  CoS  ;  but  even  then  quite  appreciable  quan- 
tities of  these  sulphides  sometimes  dissolve,  and  Ni  and  Co 
are  met  with  later  in  the  course  of  the  analysis. 

2.  A  small  black  residue  after  the  HC1  treatment  is  not 
necessarily  NiS  or  CoS,  but  it  may  be  FeS  inclosed  in  sulphur. 

3.  Moreover,  metals  of  the  copper  group,  if  incompletely 
precipitated  by  H2S,  appear  at  this  point  as  sulphides,  and 
may  be  mistaken  at  first  for  Co  or  Ni. 

4.  Action  of  aqua  regia  :  3HC1  -f  HNO3  =  C12  +  NOC1 
-\-  2H2O.     But  the   whole  of   the  chlorine   is   available   if  a 
substance  on  which  it  can  act  is  present:  3NiS  -\-  6HC1  -|-' 
2HN03  =  3NiLla  +  2NO  +  4H2Q  +  3S. 

5.  NH4OH  is  added  to  precipitate  the   iron,  which,  me- 
chanically  inclosed,    may   have    escaped   the    action    of   the 
dilute  HC1.     The  addition  of  (NH4)2S  then  shows  whether 
the   residue   consisted  entirely  of  FeS,  or  whether  NiS  and 
CoS  were  also  present. 

6.  The  borax  bead  test  generally  proves  the  presence  of 
one  or  other  of  the  two  metals.     It  is  especially  delicate  as 
a  test  for  cobalt ;   but  in  the    presence  of  much    nickel  the 
blue    color  characteristic  of  cobalt   may  be  obscured,  and  a 
negative  result  must  therefore   not  be  taken  as  evidence  of 
the  absence  of  that   metal.      If  cobalt  is  absent,  the  color 
of  the  bead    produced  in  the  oxidizing   flame  —  violet  while 
hot,  reddish  brown  when  cold  —  proves  the  presence  of  nickel. 

7.  When  either  metal  is  found  to  be  present,  the  tests  for 
the  other  must  be  made  with  great  care;  for  the  two  metals 
occur  very  commonly  together. 


34  DETECTION   OF   THE    METALS. 

8.  The  excess  of  NH4OH  in  which  the  NiO,H2  and  CoO2H2 
are  dissolved  and  the  NH4C1  must  be  completely  expelled ; 
for  nickel  is  not  completely  precipitated  by  NaOH  in  their 
presence. 

9.  The  yellow  precipitate  obtained  is  tri-potassium  cobalt/V 
nitrite  :    3KNO2.Co(NO2)3.      It  is  decomposed   by  HCi  and 
alkalies ;  it  is  decidedly  soluble  in  water,  but  almost  insolu- 
ble in  strong  KNO2  solution.     Hence  the  need  of  expelling 
the  acid  by  evaporation,  of  having  the  solution  concentrated, 
and  of  adding  a  large  excess  of  KNO2.     The  solution  must 
be  strongly  acid  with  HC2H3O2  to  set  free  the  HNO2  re- 
quired for  oxidizing  the  cobalt. 

Effect  of  the  Presence  of  Chromium  and  of  Alkaline* 
earth  phosphates  on  the  Method  of  Separation. 

General  Remarks.  —  Under  certain  circumstances  to  be  now 
described,  the  precipitate  produced  by  NH4OH  and  (NH4)2S 
may  contain  metals  of  the  alkaline-earth  group  as  well  as  of 
the  aluminum  and  iron  groups.  In  that  case  a  more  com- 
plicated process  providing  for  the  detection  of  all  these  metals 
has  to  be  used. 

Ba,  Sr,  Ca,  and  Mg  are  not  ordinarily  precipitated  by 
NH4OH  and  (NH4)2S ;  for  their  hydrates  and  sulphides  are 
soluble  in  water  (except  MgO2H2  which  dissolves  in  ammo- 
nium salts).  But  if  there  is  present  in  the  solution  an  acid 
radical  which,  combined  with  these  metals,  forms  salts  insol- 
uble in  water  and  NH4OH,  it  is  evident  that  such  salts  must 
be  held  in  solution  by  an  acid,  and  that  they  will  be  precipi 
tated  when  the  acid  is  neutralized.  Consider  two  examples ; 
CaCO3  and  Ca3(PO4)2.  Both  these  salts  are  insoluble  in 
water,  but  dissolve  in  HCI,  forming  CaCl2 ;  the  CO2,  being 
volatile,  escapes,  while  the  H3PO4  remains  in  solution.  When, 
now,  NH4OH  is  added,  no  precipitate  forms  in  the  former 
case,  but  Ca3(PO4)2  precipitates  in  the  latter. 

It  is  evident,  then,  that  it  is  unnecessary  to  try  the  pre- 
liminary test  for  phosphates  described  above  when  the  sub- 
stance dissolves  in  water  with  a  neutral  reaction,  or  in  any 
case  in  which  no  precipitate  is  produced  by  NH4OH  before 
the  addition  of  (NH4)2S  ;  for  under  these  circumstances  the 


ALUMINUM  AND  IRON  GROUPS.  35 

(NH4)2S  precipitate  can  contain  no  alkaline-earth  phosphates  ; 
nor  is  it  necessary  in  cases  where  the  substance  is  unattacked 
by  acids  and  has  been  fused  with  Na2CO8. 

Alkaline-earth  metals  are  not  necessarily  completely  pre- 
cipitated by  NH4OH  even  when  H3PO4  is  proved  to  be  pres- 
ent, for  the  amount  of  the  latter  may  not  be  sufficient  to 
combine  with  the  whole  of  the  metal.  Moreover,  if  triva- 
lent  metals  are  simultaneously  present,  the  H3PO4  combines 
with  them  by  preference,  and  the  alkaline-earth  metal  may 
be  found  wholly  or  in  part  in  the  filtrate,  there  being  no 
excess  of  H3PO4  to  combine  with  it,  or  an  insufficient  one. 
When  phosphates  are  present,  it  is  therefore  necessary  to 
look  for  alkaline-earth  metals  both  in  the  (NH4)2S  and  in 
the  (NH4)2CO3  precipitates. 

Besides  phosphates,  the  precipitate  produced  by  NH4OH 
and  (NH4)2S  may  contain  CaC2O4,  SrC2O4,  BaC2O4,  or  CaF2. 
These  are,  however,  met  with  so  rarely  in  actual  analyses, 
that  preliminary  tests  for  oxalic  and  hydrofluoric  acids  are 
dispensed  with.  But  in  case  these  acids  are  found  in  the 
subsequent  examination  for  acids,  the  (NH4)2S  precipitate 
must  be  again  analyzed  by  the  process  used  when  H3PO4  is 
present,  in  order  to  detect  the  alkaline-earth  metals  possibly 
in  combination  with  them.  Borates  of  alkaline-earth  metals 
may  also  precipitate  witn  NH4OH,  but  never  completely, 
owing  to  their  solubility  in  water  and  ammonium  salts. 

It  is  therefore  desirable  to  have  two  distinct  processes  of 
separation  of  the  aluminum  and  iron  groups :  a  short  one 
applicable  in  the  great  majority  .of  cases,  and  a  longer  one 
to  be  used  when  phosphates  of  alkaline-earth  metals  may  be 
present,  determined  by  the  above  considerations  and  by  the 
preliminary  test  for  H3PO4. 

For  an  entirely  different  reason  the  short  process  described 
below  cannot  be  used  when  chromium  is  present.  This  proc- 
ess depends  on  the  separation  of  the  metals  with  NaOH,  Fe 
and  Mn  being  precipitated,  and  Al  and  Zn  remaining  in  solu- 
tion. If,  however,  Cr  is  also  present,  Cr  and  Zn  are  precipi- 
tated with  the  Fe  and  Mn,  owing  to  the  formation  of  an  insol- 
uble double  compound  of  the  oxides  of  the  first  two  metals. 

The  presence  of  Cr  is  shown  by  the  green  or  purple  color 
?f  the  HC1  solution  of  the  (NH4)2S  precipitate.  Only  wher 


DETECTION   OF   THE  METALS. 


this  sol? von  is  colorless  should  it  be  analyzed  by  the  NaOH 
process.  Tt  is  hardly  necessary  to  add  that,  when  the  BaCOa 
process  is  used  only  on  account  of  the  presence  of  Cr,  all 
those  parts  of  It  serving  for  the  detection  or  removal  of  alka- 
line-earth metals  may  be  omitted. 

Separation  in  the  Absence  ^"  Chromium  and  of  Phosphates, 
Outline  oj*  <he  process. 

Solution  :  FeCl2,  MnC  „  ZnCl2,  A1C13. 
Boil  with  HNO%,  and  a  4  NaOH  in  excess. 


Precipitate :  FeO3H3,  MnO2H2. 


Dissolve  a  part  in 
HCl  and  add 


Precipitate : 
Fe4(Fe(CN)6)3 


Fuse  the  rest  with 


Na2MnO4  formed. 


Filtrate :  divide  into  two  parts. 


To  one  part"  add 


Precipitate:  ZnS. 


To  the  other  add 
HCl  and  NH±OH. 


Precipitate: 
A1O3H8. 


Procedure.  —  If  the  previous  tests  have  shown  the  absence 
of  phosphates  of  alkaline-earth  metals,  and  the  filtrate  from 
the  NiS  and  CoS  is  colorless,  boil  it  till  the  H2S  is  com- 
pletely  expelled;  add  HNO3  and  boil  again;  evaporate  nearly 
to  dryness,  dilute  with  a  little  water,  add  NaOH  till  the  liquid 
turns  red  litmus  paper  blue,  and  then  add  5  to  10  cc.  more. 
Filter  off  and  wash  the  precipitate. 

Notes.  —  i.  If  the  H2S  is  not  completely  expelled,  S  is 
precipitated  on  boiling  with  HNO3,  and  ZnS  may  be  precipi- 
tated on  the  addition  of  NaOH. 

2.  The  evaporation  expels  the  free  acid,  which  would 
require  NaOH  for  its  neutralization ;  and  the  use  of  an  un- 
necessary amount  of  this  reagent  is  to  be  avoided,  as  it  often 
contains  small  quantities  of  Al  or  silicic  acid  as  impurity. 


ALUMINUM  AND  IRON  GROUPS.  37 

3.  The  small  quantities  of  NiS  and  CoS  dissolved  bv  *he 
dilute  HC1  are  precipitated  in  the  form  of  hydrates  by  NaCfL 
A  precipitate  produced  by  this  reagent  may,  therefore,  consist 
entirely  of  these  metals.     Their  presence  does  not  interfere 
with  the  tests  for  Fe  and  Mn. 

4.  In  the  presence  of  much  Fe,  the  BaCO3  process  must 
be  used  to  detect  small  quantities  of  Zn ;  for  the  NaOH  pre- 
cipitate may  then  contain  even  one  per  cent,  of  Zn. 

5.  If  a  solution  containing  Cr  were  analyzed  by  this  proc- 
ess, the  Cr  would  be  found  in  the   filtrate  from  the   NaOH 
precipitate,  unless  Zn  were  also  present,  in  which  case  both 
metals  would   be   precipitated.      CrO3H3,  unlike  A1O3H3,  is 
precipitated  from  its  NaOH  solution  by  boiling. 

Procedure.  —  Test  a  portion  of  the  NaOH  precipitate  for 
Fe  by  dissolving  in  dilute  HC1,- diluting  with  water,  and 
•riding  three  or  four  drops  of  K4Fe(CN)6  solution.  Also  test 
a  part  of  the  solution  with  KSCN.  (If  Fe  is  present,  test  a 
freshly  prepared  solution  of  the  original,  substance  in  H2O  or 
HC1  with  K3Fe(CN)6  and  KCNS).  Examine  another  portion 
for  Mn  by  fusing  on  platinum  foil  with  six  parts  of  Na2CO3, 
heating  sufficiently  to  fuse  the  mixture  to  a  thin  liquid. 

Notes.  —  i.  Strong  HC1  decomposes  K4Fe(CN)6,  so  that  the 
test  for  iron  may  be  obtained  from  the  reagent  alone,  if  the 
acid  is  not  diluted.  Even  weak  acids  decompose  K4Fe(CN)6 
on  standing. 

2.  Fe4(Fe(CN)6)3    dissolves    somewhat    in    an   excess   of 
K4Fe(CN)6 ;  hence  the  addition  of  only  a  few  drops  of  the 
latter.       Most   of    the    other   metals   give    precipitates   with 
K4Fe(CN)6 ;  it   is    the   blue    color   of   the    precipitate  which 
proves  the  presence  of   iron. 

3.  By  the  fusion  Na2MnO4  is  formed,  the  oxygen  required 
being  taken    from    the  air.     Its  formation  is  indicated  by  a 
green  color  of  the  fused  mass.     The  test  is  exceedingly  deli- 
cate.     It  is  well  to  make  sure  that  the"  platinum  foil  used  is 
perfectly  clean  by  fusing  Na2CO3  on  it  before  trying  the  test. 
The  iron  remains  unchanged  as  Fe2O3. 

Procedure.  —  Test  a  part  of  the  filtrate  for  zinc  by  passing  in 
a  little  H2S.  Acidify  the  remainder  with  HC1,  make  slightly 


38  DETECTION  OF    THE    METALS. 

alkaline  with  NH4OH  ;  heat  the  solution  gently,  and  allow  it 
to  stand  half  an  hour  if  no  precipitate  is  visible  at  first. 

Notes. — i.  If  much  H2S  is  passed  in,  the  NaOH  is  changed  to 
NaSH,  and  A1O3H3  precipitates,  and  may  be  mistaken  for  ZnS. 

2.  A    dark  colored    precipitate    is  sometimes  obtained  in 
testing  for  zinc  with  H2S.     This  often  arises  from  the  pres- 
ence of  iron,  and  sometimes  from  that  of  lead  which  was  not 
completely  precipitated  by  H2S  originally,  but  was  thrown  out. 
by  (NH4)2S  and  dissolved  by  the  dilute  HC1.     To  detect  zinc 
in  such  a  case,  the  dark  precipitate  should  be  filtered  off  and 
dissolved  on  the  filter  in  a  little  hot  dilute  HC1 ;  a  few  drops 
of  HNO3  should  then  be  added,  the  solution  boiled,  NH4OH 
added  in  excess,  the  precipitate  filtered  out,  and  the  filtrate 
tested  for  zinc  with  H2S. 

3.  ZnO2H2  precipitates,  if  the  solution  is  made  just  neu- 
tral with  NH4OH.     A  slight  excess  is  necessary  to  redissolve 
it.     A  large  excess  dissolves  A1O3H3. 

4.  When  the  precipitate  of  A1O3H3  obtained  is  small,  it  is 
advisable  to  make  a  blank  experiment  with  the  same  quantity 
of  NaOH  under  exactly  the  same  conditions,  in  order  to  make 
sure  that  the  precipitate  does  not  come  from  impurity  in  that 
reagent,  or  from  the  vessels  used. 

5.  Precipitates  of  silicic  acid  are  sometimes  obtained  in 
the  test  for  Al.     For  a  method  of  removing  this  acid  at  the 
beginning  of  the  analysis,  see  page  69.     In  order  to  distin- 
guish between  A1O3H3  and  H2SiO3,  fuse  the  precipitate  on 
platinum  foil  with  KHSO4,  dissolve    the    fused  mass  in  hot 
water,  filter,  and  make  slightly  alkaline    with  NH4OH.     By 
the  fusion  the  silicic  acid  is  dehydrated  and  made  insoluble ; 
Al  passes  into  solution  in  the  form  of  A12(SO4)3. 

Separation  in  the  Presence  of  Chromium  or  of  Phosphates. 

Outline  of  the  process. 
(See  the  following  page.) 

Remark.  —  The  directions  for  that  part  of  the  following  proc- 
ess which  serves  for  the  removal  and  detection  of  phosphoric 
acid  and  the  alkaline-earth  metals  are  inclosed  in  brackets,  and 
may  be  omitted  when  these  substances  are  known  to  be  absent 
The  notes  referring  to  it  are  designated  with  an  asterisk. 


ALUMINUM  AND  IRON  GROUPS. 


39 


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4o  DETECTION  OF  THE  METALS. 

Procedure.  —  Boil  the  solution  of  the  (NH4)2S  precipitate 
in  HC1  till  the  H2S  is  completely  expelled.  [Evaporate  a 
third  of  it  just  to  dryness.  To  the  residue  add  10—20  cc. 
of  dilute  H2SO4  and  three  volumes  of  alcohol.  Allow  the 
mixture  to  stand  a  few  minutes,  filter,  and  wash  with  a  mix- 
ture of  three  volumes  of  alcohol  and  one  of  water.  Dry  the 
precipitate  and  fuse  it  on  platinum  foil  with  five  times  its 
bulk  of  Na2CO3.  Boil  the  fused  mass  with  water  till  disin- 
tegrated, filter,  and  wash  the  residue  thoroughly.  Dissolve 
it  in  dilute  HNO3,  and  examine  the  solution  for  Ba,  Sr,  and 
Ca,  as  described  on  page  46.] 

Notes.  —  i.  The  precipitate  obtained  after  the  addition  of 
(NH4)2S  may  consist  of  FeS,  MnS,  ZnS,  CoS,  NiS,  A1O3H3, 
CrO3H3,  A1PO4,  CrPO4,  Ba3(PO4)2,  Sr3(PO4)2,  Ca3(PO4)2 
MgNH4PO4,  BaC2O4,  SrC2O4,  CaC2O4,  and  CaF2 ;  also  of 
borates  of  alkaline-earth  metals,  and  silicic  acid,  if  it  has  not 
been  previously  removed.  All  these  substances  except  CaF2 
and  H2SiO3  dissolve  readily  in  dilute  HC1. 

2.*  A  portion  of  the  solution  is  tested  for  Ba,  Sr,  and  Ca, 
at  this  point,  since  BaCO3  is  to  be  added  to  the  remainder  as 
a  reagent. 

3.*  H2SO4  precipitates  Ba  and  Sr  completely  if  allowed  to 
stand  a  few  minutes.  Ca  is  also  partly  precipitated,  if  the 
solution  is  concentrated,  but  never  completely  until  alcohol 
is  added,  owing  to  its  slight  solubility  in  water. 

4.*  Fusing  with  Na2CO3  converts  the  sulphates  to  carbon- 
ates. If  the  Na2SO4  produced  is  not  washed  out  completely, 
the  sulphates  are  again  formed  on  treating  with  HNO3. 

5.*  If  (NH4)2CO3  produces  a  precipitate  in  the  filtrate 
from  the  (NH4)2S  group,  the  solution  of  that  precipitate  in 
HNO3  should  be  united  with  the  HNO3  solution  here  ob- 
tained, in  order  to  avoid  two  separate  analyses  for  the  alka- 
*  line  earth  metals. 

Procedure. — Boil  the  rest  of  the  HC1  solution  with  a 
small  quantity  of  HNO3,  dilute  a  little  of  it  and  test  with 
K4Fe(CN)6,  added  drop  by  drop,  or  with  KCNS.  (If  Fe  is 
present,  test  a  freshly  prepared  solution  of  the  original  sub- 
stance in  H2O  or  HC1  with  K3Fe(CN)6  and  KCNS.) 


ALUMINUM  AND   IRON  GROUPS.  4» 

Treat  the  remainder  as  follows:  [Add  FeCl3  little  by 
little,  till  a  drop  of  the  solution  tested  on  a  watch  glass  gives 
a  yellow  precipitate  with  NH4OH.]  Evaporate  the  solution 
to  a  small  bulk  to  expel  most  of  the  acid,  add  Na2CO3  as 
long  as  the.  precipitate  which  forms  can  be  made  to  redis- 
solve  on  shaking.  Place  in  a  small  flask,  dilute  with  water 
to  at  least  200  cc.,  cool  if  still  hot,  and  add  BaCO3,  avoiding 
a  large  excess.  Allow  the  mixture  to  stand,  with  frequent 
C'haking,  for  half  an  hour.  Filter  and  wash  the  precipitate. 

Notes.  —  \.  The  solution  tested  with  KCNS  should  be 
dilute  and  cold  ;  for  otherwise  the  HNO3  rapidly  destroys 
the  reagent.  NO2  is  thereby  formed,  which  itself  gives  a  red 
color  with  KCNS. 

2.*  If  FeCl3  were  not  added,  the  phosphates  of  the  alkaline- 
earth  metals  would  precipitate  when  the  solution  is  made 
neutral  with  BaCO3.  But  its  addition  causes  precipitation  of 
the  H3PO4  in  the  form  of  FePO4,  and  thus  allows  the  bivalent 
metals  to  pass  into  the  filtrate  in  the  form  of  chlorides. 

3.*  The  test  with  NH4OH  shows  when  sufficient  FeCl8  has 
been  added  to  combine  with  all  the  H3PO4  ;  for  before  that 
point  is  reached,  white  FePO4  precipitates  ;  but  as  soon  as  an 
excess  is  present,  brown  FeO3H3  precipitates  with  it. 

4.  BaCO3  precipitates  only  the  trivalent  metals,  aluminum, 
chromium,  and  ferric  iron,  leaving  all  the  bivalent  metals  hi 
solution. 

5.  Na2CO3,  on  the  other  hand,  being  soluble,  precipitates 
all  the  metals.     The  addition  of  enough  to  produce  a  perma- 
nent precipitate  is  therefore  fatal  to  the  separation. 

6.  The  solution  must  be  cold  ;  for  BaCO3  does  precipitate 
from  hot  solutions  certain  bivalent  metals.     These    are  also 
precipitated  if  sulphates  are  present  in  the  solution,  since  in- 
soluble BaSO4  is  thus  formed. 

7.  The  precipitate  obtained  by  following  the  above  direc- 
tions consists  of  the  excess  of  BaCO3,  of  FePO4,  of  FeO3H8, 
A1O3H8,  andCrO3H3.    Reactions  :  2FeCl3+  3BaCO8  +  3H2O 

etc. 


Procediire.  —  Dissolve   the    BaCO3   precipitate    in    dilute 
HC1,    heat   to    boiling,   and    add,    without    filtering,    dilute 


42  DETECTION  OF  THE  METALS. 

H2SO4;  allow  it  to  stand  a  few  minutes,  filter  out  the  BaSO4, 
add  to  the  filtrate  NaOH  to  strong  alkaline  reaction,  boil 
two  or  three  minutes  in  a  porcelain  dish,  filter,  and  test  the 
precipitate  for  chromium  by  fusing  on  platinum  foil  with 
Na2CO3  and  KC1O3,  boiling  the  fused  mass  with  water,  fil- 
tering, adding  HC2H3O2  to  acid  reaction,  boiling  to  expel 
CO,,  and  then  adding  Pb(C2H3O2)2.  Test  the  filtrate  from 
the  NaOH  precipitate  for  aluminum  by  acidifying  it  with 
HC1,  adding  NH4OH  till  very  slightly  alkaline,  heating  an.d 
allowing  it  to  stand,  if  no  precipitate  is  visible  at  first. 

Notes.—  i.  Dilute  HC1  is  used  for  dissolving  the  BaCO3 ; 
for  BaCl2  is  insoluble  in  strong  HC1.  The  solution  should 
be  at  the  boiling  temperature  when  H2SO4  is  added;  for  the 
BaSO4  is  then  less  likely  to  pass  through  the  filter.  A  slightly 
turbid  filtrate  need  not,  however,  be  again  filtered. 

2.  By  fusion  with  Na2CO3  and  KC1O3,  CrO3H3  is  changed 
to  Na2CrO4,  which  gives  a  yellow  color  to  the  fused  mass. 

3.  If,  however,  Mn  is  present  in  the  substance,  enough  of 
it  is  almost  always  retained  in  the  BaCO3  precipitate  to  give 
the    characteristic   green    coloration  of  Na2MnO4    on  fusing. 
The  yellow  color  of  the  Na2CrO4  is  then  obscured,  and  it  is 
necessary  to  dissolve  the  fused  mass  in  water,  to  boil  (with 
addition  of  a  few  drops  of  alcohol,  if  necessary),  in  order  to 
reduce  the  Na2MnO4  to  MnO2,  to  filter,  and  add  HC2H3O2 
and  Pb(C2H3O2)2.     The    HC2H3O2   neutralizes  the  Na2CO3, 
and  the  Pb(C2H3O2)2  precipitates  PbCrO4. 

4.  The    precipitate    produced   by    NaOH    is  usually  very- 
bulky,  owing  to  the  nresenre  of  a  large  amount  of  iron,  and 
a  common  mistake  in  testing  for  chromium  is  to  fuse  only  a 
small  portion  of  this  precipitate.     A  large  part  or,  better,  the 
whole  precipitate  should  be  employed.     It  is   best  dried  on 
the  filter,  whereby  its  volume  is  greatly  reduced,  before  mix- 
ing with  Na2CO3  and  KC1O3. 

5.  When  Cr  is  present,  small  quantities  of  it  sometimes 
precipitate  in  the  test  for  Al,  owing  to  insufficient  boiling  of 
the  NaOH  solution.     It  is  distinguished  by  the  green  color  of 
the  precipitate  after  time  has  been  allowed  for  its  separation 
from  the  liquid.     To  test  for  Al  in  it,  the  precipitate  should 


ALUMINUM  AND   IRON  GROUPS.  43 

be  filtered  off  and  dissolved  in  a  little  NaOH,  the  solution 
boiled  and  filtered,  and  the  test  for  Al  tried  in  the  filtrate. 
6.     Consult  notes  4  and  5  on  page  38. 

Procedure.  — Acidify  the  filtrate  from  the  BaCO3  precipi- 
tate with  a  few  drops  of  HC1,  and  boil  two  or  three  minutes 
to  expel  CO2.  [Make  slightly  .alkaline  with  NH4OH  and 
add  (NH4)2S,  avoiding  an  excess.  Filter  out  and  wash  any 
precipitate  that  may  form,  dissolve  it  in  HC1,  boil  till  the  H2S 
is  completely  expelled],  add  NaOH  till  alkaline  ;  filter,  test 
the  precipitate  for  Mn  by  fusion  with  Na2CO3,  and  the 
filtrate  for  Zn  by  passing  in  H2S.  [To  the  filtrate  from  the 
(NH4)2S  precipitate  add  (NH4)2CO3  and  (NH4)2C2O4;  filter; 
reject  the  precipitate  ;  concentrate  the  filtrate  as  much  as 
possible,  add  to  it  one  third  its  volume  of  NH4OH  and  a 
little  Na2HPO4,  stir,  and  allow  it  to  stand  over  night  if  no 
precipitate  appears  at  once.] 

Notes.  —  i.*  If  the  CO2  is  not  expelled,  the  alkaline-earth 
carbonates  will  be  precipitated  when  NH4OH  and  (NH4)2S 
are  added. 

2.  The  (NH4)2S  precipitate  consists  of  MnS  and  ZnS.  If 
dark  colored,  it  may  contain  small  quantities  of  NiS  and  CoSr 
which  were  dissolved  by  the  dilute  HC1  used  at  first ;  in  that 
case  it  should  be  tested  in  a  borax  bead.  FeS  will  also  be 
precipitated  here  if  the  oxidation  with  HNO3  was  incomplete. 

3  *  (NH4)2CO3  and  (NH4)2C2O4  are  added  to  remove  the 
Ba,  Sr  and  Ca,  which  would  otherwise  precipitate  with 
Na2HP04  in  the  Mg  test.  (NH4)2C2O4  is  added  since  it  pre- 
cipitates Ca  more  completely  than  (NH4)2CO3.  MgNH4PO4 
is  somewhat  soluble  in  H2O ;  hence  the  solution  must  be 
concentrated,  and  contain  a  large  quantity  of  NH4OH,  in 
which  the  precipitate  is  less  soluble. 

ADDITIONAL    REACTIONS    OF     METALS    OF    THE    ALUMINUM 
AND     IRON     GROUPS. 

Behavior  towards  the  Alkalies.  —  All  metals  of  these  groups 
are  precipitated  both  by  NaOH  and  NH4OH.  Excess  of  cold 


f6  DETECTION  OF    THE   METALS. 

2.  Ammonium  salts  considerably  increase  the  solubility  of 
the  alkaline-earth  carbonates  in  water,  so  that  small  quanti- 
ties of  the  metals  might  escape  detection  if  (NH4)2CO3  were 
alone  employed  ;  hence  the  further  tests  with  (NH4)2SO4,  and 
(NH4)oCoO4  for   Ba    and    Ca  respectively.      Even   if  already 
detected  in  the  (NH4)2CO3  precipitate,  the    traces   of  these 
metals  must  be  so  removed,  as  otherwise   they  will  precipi- 
tate in  the  test  for  Mg. 

3.  The  presence  of  ammonium  salts  is  necessary,  however, 
in  order  to  avoid  the  precipitation  of  MgCO3(NH4)oCO3.     For 
the  same  reason  the  solution  should  be  moderately  dilute  and 
not  stand  long  before  filtering. 

Procedure.  —  Dissolve  the  precipitated  carbonates  in  a 
small  quantity  of  dilute  HNO3.  Evaporate  the  solution  to 
dryness  in  a  small  porcelain  dish,  and  heat  quite  strongly  on 
an  iron  plate  until  no  odor  of  HNO3  can  be  detected.  As 
soon  as  it  is.  cold,  rub  the  contents  of  the  dish  to  powder  by 
means  of  a  pestle,  add  5-10  cc.  of  a  mixture  of  equal  vol- 
umes of  absolute  alcohol  and  ether,  and  triturate  with  the 
pestle  for  two  or  three  minutes.  Filter,  and  add  two  drops 
of  dilute  H2SO4  to  the  filtrate.  Wash  the  residue  on  the 
filter  with  small  quantities  of  the  alcohol-ether  mixture  until 
the  wash  water  shows  no  turbidity  with  a  drop  of  dilute  sul- 
phuric acid ;  then  dissolve  it  in  10-20  cc.  of  water,  and  filter 
if  not  perfectly  clear.  To  a  third  of  the  solution  add  two  or 
three  drops  of  acetic  acid,  dilute  it  with  50  cc.  of  water, 
heat  to  boiling,  and  add  gradually  K2CrO4  until  present  in 
slight  excess.  If  no  precipitate  forms,  add  NH4OH  and 
(NH4)2CO3  to  the  remaining  two  thirds  of  the  aqueous 
solution.  If  a  precipitate  forms,  treat  the  remainder  of 
the  solution  in  just  the  same  way  as  the  first  third  was 
treated,  .unite  the  two  portions,  heat  to  boiling,  and  filter. 
Then  add  to  the  filtrate  NH4OH  and  (NH4)2CO3. 

Notes.  —  i.  In  order  that  the  separation  of  the  nitrates  by 
the  alcohol-ether  mixture  may  be  complete,  it  is  essential  that 
they  be  completely  anhydrous;  for  Sr(NO3)2  is  very  readily 


ALKALINE- EARTH  GROUP. 


47 


soluble  in  water,  and  Ba(NO3)2  moderately  so.  The  temper- 
ature, therefore,  must  be  sufficiently  high  and  long  continued 
to  effect  dehydration  ;  it  may  reach  180°  without  injury,  but 
too  high  a  temperature  would  convert  the  nitrates  to  oxides. 
Since  the  nitrates  are  hygroscopic,  they  must  not  be  left  ex- 
posed to  the  air ;  but  they  should  be  treated  immediately  with 
the  solvent. 

2.  If  the  precipitate  produced  by  H2SO4  is  very  small,  it 
may  be  due  to  a  trace  of  Sr(NO3)2  dissolved,  owing  to  the 
presence    of    water,  by  the   alcohol-ether  mixture.      In   that 
case,  therefore,  collect  it  upon  a  small  filter,  pour  through  the 
latter  repeatedly  5  cc.  of  a  hot,  concentrated  (25  per  cent.) 
solution  of  (NH4)..>SO4,  to  which  a  few  drops  of  NH4OH  have 
been  added,  make  barely  acid  with  acetic  acid,  and  add  a  few 
drops  of  (NH4)oC2O4.     The  formation  of  a  precipitate  shows 
conclusivel}   the   presence   of    Ca,  for  the  minute  quantities 
of    SrSO4    soluble    in    (NH4)2SO4    are    not    precipitated    by 
(NH4).2C,O4. 

3.  In  order  that  the  precipitation  of  BaCrO4  may  be  com- 
plete, the  quantity  of  free  HC2HSO2  present  must  be  small 
The  solution  is  diluted  to  prevent  the  precipitation  of  stron- 
tium.    It  is  precipitated  hot,  since  otherwise  BaCrO4  is  liable 
to  pass   through   the  filter.      A   perfectly  clear  filtrate   must 
always  be  obtained  before  adding  (NH4)2CO3. 

4  In  order  to  detect  minute  quantities  of  Ba,  Sr,  and  Ca, 
it  is  advisable  to  examine  the  nitrates  with  the  spectroscope 
before  treating  them  with  the  alcohol-ether  mixture. 

Procedure.  —  Evaporate  the  filtrate  from  the  (NH4)2CO3 
(or  (NH4)2SO4  and  (NH4)2C2O4)  precipitate  in  a  porcelain 
vessel  till  ammonium  salts  begin  to  crystallize  out.  Acidify 
the  liquid  with  HC1,  filter,  and  add  to  a  portion  of  it  a  third 
its  volume  of  NH.OH  and  some  Na2HPO4.  Rub  the  sides 
of  the  tube  with  a  glass  rod,  and  if  no  precipitate  appears, 
allow  it  to  stand  over  night. 

Evaporate  the  remainder  of  the  solution  to  dryness  in  a 
small  porcelain  disb,  and  ignite  the  residue  not  above  a  faint 
red  heat  until  no  more  white  fumes  come  off,  taking  care  to 
heat  the  sides  as  well  as  the  bottom  of  the  dish.  Dissolve 


48  DETECTION  OF  THE  METALS. 

in  a  little  water,  filter,  and  evaporate  to  dryness  in  a  small 
beaker ;  note  the  quantity  of  the  residue,  and  introduce  a 
little  of  it  into  a  flame  on  a  platinum  wire.  Dissolve  in  the 
least  possible  quantity  of  water,  add  a  few  drops  of  H2PtClG, 

stir,   and   allow  the  solution  to  stand. 

• 

Notes. —  i.  Concerning  the' precipitation  of  MgNH4PO4 
see  note  4,  page  43.  All  other  metals  except  arsenic  and 
the  alkalies  are  also  precipitated  by  Na2HPO4 ;  so  the  mag- 
nesium test  can  be  made  only  after  they  are  removed.  The 
precipitate  should  be  crystalline ;  if  flocculent  it  may  consist 
of  A1PO4,  present  here  by  reason  of  the  solubility  of  A1O3H3 
in  NH4OH  and  the  use  of  too  large  a  quantity  of  that  re- 
agent in  precipitating  the  aluminum  and  iron  groups. 

2. ,  Presence  of  MgCl2  does  not  interfere  with  the  H2PtCl6 
test  tor  potassium.  When  present  in  large  amount,  however, 
it  is  sometimes  desirable  to  remove  it,  in  order  to  form,  from 
the  amount  of  residue  left  after  ignition,  an  approximate  idea 
of  the  quantity  of  alkali  metals  present.  It  is  removed  by 
adding  BaO2H2  to  the  solution  from  which  the  ammonium 
salts  have  been  expelled,  boiling,  filtering  out  the  MgO2H2, 
precipitating  the  barium  from  the  filtrate  with  NH4OH  and 
(NH4)2£O3,  filtering,  evaporating  the  filtrate,  and  igniting  the 
residue  to  expel  ammonium  salts.  By  this  same  method  all 
other  metals  (except  arsenic)  can  be  removed  from  a  solution 
to  be  tested  for  alkalies. 

3.  Since  ammonium  salts,  like  those  of  potassium,  give  a 
precipitate  with  H2PtCl6,  great  care  must  be  taken  to  remove 
them  completely ;  ignition  above  faint  redness,  however,  vol- 
atilizes KC1  and   NaCl. 

4.  No  satisfactory  precipitant  for  sodium  is  known.     The 
flame  test  is  so  exceedingly  delicate  that  only  when  the  yellow 
color  is  intense  and  persistent  is  it  to  be  taken  as  an  indica- 
tion  of  sodium   in  appreciable  quantity.      At  best,   however, 
the  flame  test  is  very  unsatisfactory ;    it  is  therefore  always 
advisable  to  determine  approximately,  as  directed  in  the  Pro-   • 
cedure,  the  amount  of  residue  left  after  expulsion  of  the  am- 
monium salts ;  for,  in  this  way,  an   idea  can  be  formed  of  the 
amount  of  Na  present,  when  Mg  and  K  are  absent,  or  when 


ALKALI  GROUP.  49 

present  only  in  small  amounts.  If  Mg  or  K  are  present  in 
considerable  amounts,  it  is  even  usually  advisable  to  remove 
them  by  precipitation,  and  to  determine  the  amount  of  Na  in 
the  filtrate  by  evaporation.  Magnesium  is  best  removed  as 
described  in  note  2  above.  To  remove  potassium,  add  H2PtCl6 
in  the  usual  manner  ;  evaporate  the  filtrate  to  dryness,  ignite 
the  residue  at  a  temperature  below  redness  and  treat  it  with 
water,  filter  the  solution  and  finally  evaporate  it  to  dryness. 
By  the  ignition  the  Na2PtCl6  and  H2PtCl6  are  destroyed,  Pt 
and  NaCl  alone  remaining  in  the  dish. 

5.  K3PtCl6  is  soluble  in  100  parts  of  cold  and  20  parts  of 
hot  water ;  hence  the  need  of  having  the  solution  cold  and 
very  concentrated,  and  of  allowing  it  to  stand. 

6.  Very  small  quantities  of  K  and  Na,  like  those  Ba,  Sr, 
and  Ca,  are  best  detected  with  the  aid  of  the  spectroscope. 
This  method  does  not  answer  the  ordinary  purposes  of  anal- 
ysis, however;  for  it  gives  but  little  idea  of  the  quantity  of 
the  element  present,   and  does  not  distinguish  unimportant 
traces  from  considerable  amounts. 

Procedure.  —  Test  a  portion  of  the  original  solid  substance 
for  ammonium  salts  by  mixing  it  in  a  small  beaker  with  solid 
CaO2H2  moistened  with  a  little  water.  Cover  the  beaker 
with  a  watch  glass  with  a  piece  of  moist  red  litmus  paper 
adhering  to  the  under  side,  and  heat  gently. 


DETECTION    OF    THE    ACIDS. 


GENERAL     REMARKS. 

The  number  of  acids  to  be  tested  for  is  often  restricted  by 
the  solubility  of  the  substance  taken  in  connection  with  the 
metals  already  found  in  it.  A  substance  completely  soluble 
in  water  containing  a  certain  metal  cannot  contain  any  acid 
known  to  form  an  insoluble  salt  with  that  metal ;  for  example, 
a  barium  salt  soluble  in  water  need  not  be  tested  for  sulphate, 
chromate,  phosphate,  oxalate,  borate,  carbonate,  or  fluoride; 
a  barium  salt  soluble  in  dilute  acids  cannot  contain  sulphate. 
But,  on  the  other  hand,  when  the  substance  is  insoluble,  it  is 
not  safe  to  conclude  that  all  acids  forming  soluble  salts  with 
the  metals  present  are  therefore  absent;  for  the  solution  of 
such  salts  may  be  prevented  by  the  presence  of  insoluble  sub- 
stances which  hold  them  back  mechanically  or  by  reason  of 
chemical  combination ;  for  example,  Ba(NO3)2  is  retained  in 
this  way  by  BaSO4.  Moreover,  many  salts  which  when  normal 
are  readily  soluble  are  insoluble  when  basic  ;  for  example, 
basic  lead  nitrate,  Pb(OH)NO3. 

In  order  to  determine  what  acids  are  excluded  by  the  solu- 
bility of  the  substance,  the  solubility  of  the  various  salts  of 
the  metals  present  must  be  considered.  The  following  brief 
statement  will  be  of  assistance  : 

1.  All  K,  Na,  and  (NH4)  salts  are  soluble  in  water  (except 
K2PtCl6,  (NH4)2PtCl6,  KHC4H406,  and  (NH4)HC4H4O6). 

2.  All  nitrates,  chlorates,  and  acetates  are  soluble  in  watet 
(except  certain  basic  nitrates  and  acetates). 

3.  All   carbonates,   phosphates,  borates,  oxalates,  arseni- 
ates,  and  arsenites,  except  those  of  the  alkalies,  are  insoluble 
or  only  slightly  soluble  in  water,  but  are  readily  soluble  in 
dilute  acids. 

4.  All  chlorides,  bromides,  and  iodides,  except  the  lead, 
silver,  and  mercurous  compounds  and  HgI2,  and  all  sulphates, 
except  those  of  Ba,  Sr,  Ca,  and  Pb,  are  soluble  in  water. 


DIVISION  INTO    GROUPS.  5 

The  number  of  acids  to  be  tested  for  is  further  limited  in 
cases  by  the  nature  of  the  substance  analyzed.  For 
example,  it  is  useless  to  test  a  mineral  for  organic  and  cyan- 
ogen acids,  or,  in  general  if  insoluble  in  water,  for  nitrates, 
chlorates,  bromides,  and  iodides;  alloys  contain  no  acids;  etc. 

DIVISION    OF    THE    ACIDS    INTO    GROUPS. 

The  detection  of  the  acids  does  not  require,  like  that  of 
the  metals,  their  separation  from  one  another.  They  are, 
nevertheless,  divided  into  groups  according  to  their  behavior 
toward  certain  general  reagents,  which,  however,  serve  to  show 
the  presence  or  absence  of  a  whole  group  of  acids,  and  not 
to  separate  one  group  from  another. 

The  acids  are  primarily  divided  into  two  classes : 

1.  Those  whose  salts  blacken  and  emit  a  burnt  odor  when 
ignited  in  a  closed  tube ;  that  is,  the  organic  acids. 

2.  Those  whose  salts  do  not  behave  thus ;  that  is,  the  inor- 
ganic acids. 

Notes. —  i.  Blackening  does  not  necessarily  prove  an 
organic  acid.  Starch,  sugar,  and  other  organic  substances 
also  char. 

2.  Blackening,  unaccompanied  by  a  burnt  odor,  is  not  due 
to  organic  matter.    Cu,  Co,  and  Ni  salts  may  cause  it,  owing  to 
expulsion  of  the  acid  and  change  to  the  black  oxides. 

3.  The  presence  of  organic   matter  is  generally  best  de- 
tected by  heating  the  substance  suddenly  to  a  high  temperature. 

4.  Oxalic  acid,  an  organic  acid,  is  here  classed  with  the 
inorganic ;  for  its  salts  do  not  char,  or  char  but  slightly. 

Organic  Acids.  —  These  are  exceedingly  numerous,  and  no 
attempt  to  group  them  separately  will  be  made.  Acetic  and 
tartaric  acids,  the  two  most  commonly  met  with,  are  the  only 
ones  that  will  be  considered  in  this  course. 

Inorganic  Acids.  —  These  including  the  two  organic  acids, 
acetic  and  tartaric,  are  divided  into  groups  as  follows : 

GROUP  I.  Sulphuric  Acid  Group.  —  Acids  precipitated  from 
neutral  solution  by  BaCl2.  They  are :  H2SO4,  H2CrO4,  H3PO4, 
H3B03,  H2C204,  HF,  H2C03)  H4SiO4,  H2C4H4O«.  (HaSO,, 
H2S203,  H3As03,  H3As04.) 


5*  DETECTION  OF  THE  ACIDS. 

GROUP  II.  Halogen  Group.  —  Acids  precipitated  from  dilute 
HNO3  solution  by  AgNO3.  They  are  :  HC1,  HBr,  HI,  HCN, 
H2S,  H3Fe(CN)6,  H4Fe(CN)6. 

GROUP  III.  Nitric  Acid  Group.  —  Acids  not  precipitated  by 
BaCl2  nor  by  AgNO3.  They  are :  HNO3,  HC1O3,  HC2H3O2. 

Note.  —  Though  AgNO3  precipitates  none  of  the  acids  of 
the  first  group  from  an  acid  solution,  it  precipitates  all  of  them 
except  H2SO4  and  HF  from  a  neutral  solution.  The  acids  of 
the  second  group  are  also  precipitated  by  AgNO3  in  neutral 
solution.  From  an  acid  solution  BaCl2  precipitates  only 
H2SO4. 

GENERAL  TESTS. 

Procedure.  —  If  the  solution  is  acid,  make  a  portion  of  it 
slightly  alkaline  with  NH4OH  ;  if  it  is  alkaline  with  Na2CO3, 
acidify  a  portion  with  HNO3,  boil  it  a  few  minutes  to  expel 
CO2,  and  make  it  alkaline  with  NH4OH  ;  add  BaCl2  and 
CaCl2,  and  if  no  precipitate  forms  at  once,  allow  the  solution 
to  stand  some  minutes.  If  a  precipitate  forms,  add  HC1  in 
considerable  quantity. 

Notes.  —  i.  Non-formation  of  a  precipitate  in  neutral  solu- 
tion proves  the  absence  of  all  acids  of  the  first  group  except 
H3BO3,  which  is  precipitated  by  BaCl2  and  CaCl2  only  from 
rather  concentrated  solutions. 

2.  This  conclusion  is  correct,  however,  only  when  no  con- 
siderable quantity  of  ammonium  salt  is  present ;  for,  with  the 
exception  of  BaSO4  and  CaC2O4  all  these  precipitates  dissolve 
in  ammonium  salts  somewhat,  and  some  of  them  (the  borate, 
fluoride,  and  tartrate)  dissolve  quite  readily. 

3.  If  the  precipitate  dissolves  completely  in  HC1,  H2SO4 
is  absent ;  if  it  is  not  completely  soluble,  H2SO4  is  present, 
and  other  acids  of  its  group  may  also  be  present.      Owing 
to  the  solubility  of  their  Ba  and  Ca  salts  in  ammonium  salts, 
their  presence  or  absence  cannot  be  determined  by  filtering 
and  neutralizing  the  filtrate  from  the  BaSO4  with  NH4OH. 

4.  The  precipitates  are  all  white  except  the  chromate. 

5.  CaCl2  is  added,  since  CaC2O4,  CaF2,  and  CaC^O,* 
are  more  insoluble  than  the  corresponding  barium  salts. 


GENERAL    TESTS. 


53 


Procedure.  —  Acidify  a  portion  of  the  solution  with  HNO8, 
and  add  AgNO3.  If  a  precipitate  forms,  filter  it  off  and 
test  the  filtrate  with  more  AgNO3  to  insure  complete  precip- 
itation. Then  carefully  pour  a  little  dilute  ammonia  down 
the  sides  of  the  tube  so  as  to  form  a  layer  on  top.  Note 
whether  a  precipitate  forms  at  the  junction,  and  what  its 
color  is. 

Notes.  —  i.  Non-formation  of  a  precipitate  in  acid  solution 
proves  the  absence  of  the  halogen  group.  If  no  precipitate 
is  obtained  in  neutral  solution,  all  acids  of  the  sulphuric  acid 
group  except  H2SO4  and  HF  are  absent. 

2.  The   color  of  the  silver  precipitates  taken  in  connec- 
tion with  their  solubility  in  HNO3  is  an  important  indication. 
Agl,  Ag3PO4,  and  Ag3AsO3  are  yellow;    AgBr  is  yellowish 
white;  Ag3AsO4  and  Ag3Fe(CN)6  are  brownish  red;  Ag2CrO4 
is  dark  red;    Ag2S   is  black,  and  the   remainder  are   white. 
Ag2CO3,  white  at  first,  rapidly  turns  yellow,  and  on  boiling, 
brown   owing  to  change  to  Ag2O. 

3.  All  the  precipitates  with  the  exception  of  Agl,  Ag2S  and 
Ag4Fe(CN)6  dissolve  in  NH4OH,  most  of  them  very  readily; 
AgBr  dissolves  only  sparingly.     Therefore  AgNO3  precipitates 
chromates,  phosphates,  etc.,  only  from  a  perfectly  neutral  solu- 
tion.    As  the  whole  solution  is  not  easily  made  exactly  neu- 
tral, the  NH4OH  is  added  on  top,  by  which  a  neutral  layer 
is  formed  at  the  junction. 

4.  Ag2C2O4  dissolves  in  HNO3  with  some  difficulty ;  hence, 
if  considerable  HNO3  is  not  added,  it  might  be  mistaken  for 
a  halogen  compound. 

5.  A  grayish  brown  precipitate  of  Ag2O,  very  soluble  in 
HNO3,  NH4OH,   and   NH4NO3,  may  form,  even   when   no 
acids  of  the  first  and  second  groups  are  present. 

Procedure.  — Add  a  few  drops  of  strong  H2SO4  to  some 
of  the  finely  powdered  substance  placed  in  a  very  small 
test-tube,  and  heat  gently  —  not  enough  to  cause  vola- 
tilization of  the  H2SO4. 


54  DEFECTION  OF  THE  ACIDS. 

Notes.  —  i.  Salts  of  the  various  acids  give  indications  as 
follows : 

Chromates,  sulphates,  phosphates,  and  borates  remain 
unchanged. 

Silicates  remain  unchanged,  or  decompose  with  separation 
of  silicic  acid. 

Sulphites  evolve  SO.2,  recognized  by  its  odor. 

Thiosulphates  evolve  SO2  and  give  a  precipitate  of  sulphur. 

Oxalates  and  carbonates  effervesce,  owing  to  escape  of  CO, 
(accompanied  by  CO  in  the  case  of  oxalates). 

Fluorides  evolve  HF,  which  etches  the  watch  glass  on 
which  the  test  is  tried. 

Chlorides  evolve  HC1,  recognized  by  its  odor. 

Bromides  and  iodides  evolve  Br  and  I,  recognized  by  their 
color  and  odor. 

Cyanides  evolve  HCN,  recognized  by  its  odor. 

Sulphides  evolve  H2S,  known  by  its  odor  and  reaction  with 
Pb(C2H302)2. 

Nitrates  evolve  HNO3,  recognized  by  its  odor. 

Chlorates  evolve  C1O2,  a  green  gas  which  smells  like  chlo- 
rine and  colors  the  I12SO4  intensely  yellow. 

Acetates  evolve  HC2H3O2,  known  by  its  odor. 

Tartrates  char  and  cause  blackening  of  the  H2SO4. 

2.  These  indications  should  not  be  taken  as  conclusive, 
but  should  be  confirmed  by  appropriate  special  tests.  This 
is  especially  true  where  the  results  are  negative,  owing  to  the 
fact  that  certain  insoluble  salts  are  not  decomposed  as  above 
stated. 

SPECIAL  TESTS. 

By  the  considerations  stated  on  pages  50  and  51,  and  by  the 
BaCl2  and  AgNO3  tests,  the  examination  for  acids  is  much 
simplified.  Moreover,  in  the  analysis  for  metals,  the  pres- 
ence of  many  acids,  namely:  H2CO3,  H2S,  HCN,  H2SO3, 
H2S2O3,  HC1O3,  on  addition  of  HC1 ;  H3AsO3,  H3AsO4, 
H2CrO4,  by  the  action  of  H2S ;  —  will  generally  have  been 
demonstrated.  For  the  acids  not  already  detected,  or  not 
proved  absent,  special  tests  must  be  tried,  as  described  below, 


SULPHURIC  ACID   GROUP.  55 

SULPHURIC    ACID    GROUP. 

Chromic  Acid. 

1.  Reducing  agents,  such  as  nascent  hydrogen,  strong  HC1, 
H2S  in  presence  of  acid,  reduce  chromates  to  chromium  salts, 
the  color  of  the  solution  changing  from  yellow  or  red  to  green. 
In  the  presence  of  insufficient  acid  H2S  effects  only  partial 
reduction,  producing  a  brown  precipitate  of  hydrated  CrO2, 
which,  in  the   analysis  for  metals,  might  be   mistaken   for  a 
sulphide  of  the  H2S  group. 

2.  Pb(C2H3O2)2  precipitates  PbCrO4,  insoluble  in  HC2H3O2. 

3.  Acids   change  the  yellow  color  of   solutions  of  normal 
chromates  to  red,  owing  to  formation  of  dichromates. 

Sulphuric  Acid. 

The  BaCl2  test  in  presence  of  HC1  is  conclusive  for  this 
acid.  It  is  simply  necessary  to  avoid  the  addition  of  much 
HC1;  for  this  prevents  the  precipitation  of  small  quantities  of 
BaSO4,  and  may  cause  the  precipitation  of  BaCl2. 

Sulphurous  and  Thiosulphuric  Acids. 

These  are  detected  by  the  action  of  H2SO4  on  the  solid 
substance  or  on  a  concentrated  solution  of  it. 

Phosphoric  Acid. 

Procedure.  —  Place  3  or  4  cc.  of  (NH4)2MoO4  solution  in 
a  test  tube,  and  add  to  it  not  more  than  half  as  much  of  the 
solution  to  be  tested,  having  first  acidified  the  latter  with 
HNO3.  Allow  the  solution  to  stand,  if  no  precipitate  forms 
at  once. 

Notes.  —  i.  The  (NH4)2MoO4  is  used  in  excess;  for  the 
precipitate  is  less  soluble  in  it  than  in  pure  water.  Dilute 
HNO3  and  NH4NO3  also  diminish  its  solubility;  but  HC1 
and  chlorides  present  in  considerable  quantity  render  the  test 
less  delicate.  In  alkaline  solution  the  precipitate  does  not 
form. 

2.  The  yellow  precipitate  is  ammonium  phospho-molybdate 
of  complicated  and  somewhat  variable  composition  (approxi 
mately  (NH4)3PO4.12MoO3). 


56  DECTECTION  OF  THE  ACIDS. 

Procedure.  —  Add  enough  NH4OH  to  fora?  one  third  cht 
bulk  of  the  solution,  and  then  a  little  magnesia  mixture 
(MgCl2,  NH4C1,  and  NH4OH).  Stir  with  a  glass  rod  and 
allow  the  mixture  to  stand  for  some  hours. 

Notes.  —  i.  The  NH4C1  is  added  to  the  reagent  to  prevent 
the  precipitation  of  MgO2H2. 

2.  H3AsO4  gives  a  similar  precipit&U.  (See  Note  5,  page  25.) 
If  present,  it  is  first  removed  by  passing  H2S  into   the   hot 
acidified  solution. 

3.  The  test  is  only  applicable  when  NH4OH  alone   pro- 
duces no  precipitate.     If  metals  precipitated  by  NH4OH  are 
present,  the  (NH4)2MoO4  test  must  be  used. 

Boric  Arid. 

Procedure.  —  Add  to  the  solid  substance,  obtained  if  neces- 
sary by  evaporating  the  solution,  concentrated  H2SO4  and 
C2H5OH.  Warm  the  mixture,  set  fire  to  it,  and  note  whether 
the  borders  of  tne  flame  are  colored  green. 

Notes.  —  i.  The  color  is  best  seen  at  the  moment  when  the 
alcohol  is  lighted. 

2.  Copper,  if  present,  must  be  first  removed  with  H2S. 

3.  In  the  presence  of  chlorides  the  test  is  unreliable  ;  for 
ethyl  chloride,  which  also  tinges  the  flame  green,  is  formed  by 
their  action  on  the  alcohol. 

4.  If  a  solution  to  be  tested  has  to  be  evaporated,  it  must 
first  be  made  alkaline,  since  H3BO3  is  volatile  with  steam. 

Procedure. — Acidify  the  solution  slightly  with  HC1,  and 
partly  dip  a  piece  of  turmeric  paper  in  it,  and  dry.  Note 
whether  the  dipped  part  assumes  a  red  tint. 

Note.  — If  the  HC1  is  not  very  dilute,  it  imparts  a  brownish 
black  color  to  the  turmeric  paper,  which  obscures  the  H3BO3 
test. 

Oxalic  Acid. 

Procedure.  —  If  the  solution  is  neutral  or  alkaline,  acidify 
with  HC2H3O2,  add  CaSO4,  and  allow  it  to  stand.  If  the 
solution  is  acid  with  HC1  or  HNO3,  first  add  a  considerable 
quantity  of  NaC2H8O2,  and  then  add  CaSO4. 


SULPHURIC  ACID   GROUP.  57 

. —  i.  CaC2O4  is  nearly  as  insoluble  in  HC2H3O2  as 
in  H2O.  The  presence  of  that  acid  is  necessary  to  prevent 
the  precipitation  of  other  calcium  salts,  such  as  CaCO3, 
Ca3(PO4)2.  CaF2,  however,  is  also  insoluble  in  HC2H3O2. 

2.  Mineral  acids  dissolve  CaC3O4.  NaC2H3O2  neutralizes 
them  with  formation  of  their  sodium  salt  and  HC2H3O2,  and 
is  often  employed  for  this  purpose  in  other  cases. 

Hydrofluoric  Acid. 

This  is  commonly  met  with  only  in  minerals  and  metal-, 
lurgical  products. 

Procedure.  — ,  Mix  the  dry,  finely  powdered  substance  in  a 
platinum  crucible  or  on  platinum  foil  with  enough  strong 
H2SO4  to  make  a  thin  paste.  Cover  it  with  a  watch  glass 
coated  on  its  convex  face  with  beeswax  through  which  some 
markings  have  been  made  with  a  pointed  piece  of  wood. 
Fill  the  watch  glass  with  cold  water,  and  warm  the  mixture 
very  gently,  taking  care  not  to  melt  the  wax.  After  half  an 
hour,  melt  off  the  wax  and  examine  the  glass  for  etchings. 

Notes.  —  i.  In  order  to  detect  small  quantities  of  fluorine, 
a  large  excess  of  H2SO4  should  be  avoided,  the  watch  glass 
should  fit  the  crucible  or  foil  closely,  and  considerable  time 
should  be  allowed.  By  breathing  on  the  watch  glass  slight 
etchings,  not  otherwise  noticeable,  are  made  apparent. 

2.  The  etching  is  due  to  the  action  of  HF  on  the  silicate 
of  the  glass,  SiF4,  a  gas,  being  formed. 

3.  If  the  substance  tested  is  not  decomposed  by  H2SO4,  or 
if  it  itself  contains  SiO2  or  silicate,  it  is  evident  that  the  test,  if 
negative,  is  not  conclusive.     Another  process,  described  on 
page  83,  is  then  followed. 

Carbonic  Acid. 

Procedure.  —  Cover  some  of  the  finely  powdered  substance 
in  a  test  tube  with  a  little  water ;  boil  to  expel  the  air,  and 
add  HC1.  If  bubbles  of  gas  escape,  hold  a  drop  of  BaO2H2 
caught  on  the  end  of  a  glass  rod  just  above  the  surface  of 
the  liquid,  and  note  whether  it  becomes  turbid. 


5  8  DETECTION    OF    THE    ACIDS. 

Silicic  Acid. 

Procedure. — Add  HC1  to  the  solution,  evaporate  it  to  dry- 
ness,  and  ignite  the  residue  at  125°  in  a  hot  closet  for  an 
hour  or  more.  Moisten  with  strong  HC1,  warm,  add  water, 
and  heat  to  boiling.  If  a  residue  remains  undissolved,  trans- 
fer it  to  platinum  foil,  add  carefully  one  or  two  drops  of  HF 
and  a  drop  of  H2SO4,  and  heat  until  the  acid  is  volatilized. 

Notes. —  i.     For  explanation  see  note  4,  page  73. 

2.  If  the  residue  consists  only  of  SiO2,  it  will  completely 
volatilize  on  treatment  with  HF,  owing  to  the  formation  of 
gaseous  SiF4.  In  using  HF  take  the  greatest  care  not  to  get  it 
on  the  hands  nor  to  breathe  it. 

HALOGEN  GROUP. 

Hydrochloric  Acid. 

The  test  relied  upon  in  most  analyses  for  the  detection  of 
this  acid  is  simply  the  formation  with  AgNO3  of  a  white  pre- 
cipitate insoluble  in  HNO3,  other  halogens  being  proved  ab- 
sent by  the  special  tests  for  them.  This  method  is  simpler 
and  far  more  delicate  than  to  try  special  tests  for  chlorine. 
Such  tests  have  to  be  resorted  to,  however,  in  some  cases, 
when  other  halogens  are  present. 

Hydrobromic  Acid. 

Procedure.  —  Add  to  the  neutral  or  slightly  acid  solution 
enough  CS2  to  form  a  large  drop  at  the  bottom  of  the  test 
tube  ;  then  add  a  few  drops  of  chlorine  water,  and  shake  the 
tube. 

Note.  —  The  CS2  is  added  to  dissolve  the  liberated  bromine, 
thus  making  the  test  more  delicate.  The  color  is  reddish 
brown  if  much  bromine  is  present  ;  yellow  if  only  a  little.  If 
care  is  not  taken  to  avoid  an  excess  of  chlorine,  the  solu- 
tion will  be  decolorized,  owing  to  formation  of  bromine  chlo- 
ride, BrCl.  Make  sure  that  the  chlorine  water  used  smells 
strongly  of  the  gas. 


HALOGEN  GROUP  59 

Hydriodic  Acid. 

Procedure.  —  Make  the  solution  distinctly  acid  with  HClf 
add  a  drop  of  KNO2  solution  and  enough  CS2  to  form  a  large 
drop  at  the  bottom  of  the  test  tube,  and  shake  vigorously. 

Note.  —  The  HNO2  liberates  iodine  and  is  itself  reduced  to 
NO.  Chlorine  water  (as  in  the  test  for  HBr)  answers  equally 
well  in  most  cases,  but  is  not  so  suitable  for  the  detection  of 
very  small  quantities  of  HI,  since  if  used  in  excess  it  oxidizes 
the  liberated  iodine,  thus  destroying  its  color. 

Hydrocyanic  Acid. 

Procedure,  —  If  the  solution  is  acid,  make  it  alkaline  by 
the  addition  of  NaOH  ;  then  add  solution  of  FeSO4  and  a 
few  drops  of  FeCl3.  Heat  the  mixture  for  a  short  time,  and 
then  acidify  with  HC1. 

Notes. —  i.  On  the  addition  of  the  iron  salts  a  greenish 
black  precipitate  of  ferrous-ferric  hydrate  forms,  even  in  the 
absence  of  HCN ;  but  this  dissolves  when  HC1  is  added,  the 
insoluble  Prussian  blue  then  becoming  visible.  If  only  traces 
of  HCN  are  present,  the  liquid  simply  appears  green  at  first 
and  a  slight  blue  precipitate  separates  on  long  standing. 

2.     The  reactions  in  this  process  are : 

FeSO4  +  6KCN  =  K4Fe(CN)6  +  K2SO4 

3K4Fe(CN)6  +  4FeCl3  =  Fe4(Fe(CN)6)3  +  12KC1. 

Procedure.  — If  the  solution  is  acid,  make  it  alkaline  with 
NH4OH,  and  add  one  or  two  drops  of  (yellow)  (NH4)2SX. 
Heat  on  the  water  bath  till  a  drop  of  the  solution  no  longer 
reacts  with  Pb(C2H3O2)2 ;  then  acidify  with  HC1  and  add 
a  little  FeCl3. 

Note.  —  The  reaction  is  : 
(NH4)2SX  +  (x  —  1)  KCN  =  (NH4)2S  +  (*  —  !)  KCNS. 

Hydroferrocyanic  and  Hydrofefricyanic  Acids. 
Procedure.  —  Add  to  one  portion  of  the  slightly  acid  solu- 
tion a  few  drops  of  FeSO4,  and  to  another  portion  a  few 
drops  of  FeCl3. 


6o  DETECTION  OF   THE  ACIDS. 

Hydrogen  Sulphide. 

Procedure. — Warm  some  of  the  finely  powdered  sub- 
stance in  a  test  tube  with  dilute  HC1,  insert  a  piece  of 
Pb(C2H3O2)2  paper  in  the  top  of  the  tube,  and  if  no  blacken- 
ing appears,  add  some  pure  powdered  zinc ;  cork  the  tube 
loosely,  and  allow  it  to  stand. 

Notes.  —  i.  Some  sulphides  (for  example,  HgS,  FeS2,  etc.) 
are  not  decomposed  by  the  acid  alone.  The  nascent  hydro- 
gen evolved  from  the  zinc  reduces  these,  however,  with  evolu- 
tion of  H2S. 

2.  If  sulphide  is  present  in  considerable  quantity,  it  is 
detected  in  the  preparation  of  the  solution  for  analysis,  either 
by  the  odor  of  H2S,  if  the  substance  is  dissolved  in  HC1,  or  by 
the  separated  sulphur,  if  HNO3  or  aqua  regia  is  used. 

ACIDS  OF  THE  HALOGEN  GROUP  IN  THE  PRESENCE 
OF  EACH  OTHER. 

Iodides,  cyanides,  and  sulphides  are  readily  detected  by 
the  tests  above  described,  even  in  the  presence  of  other 
acids  of  the  group. 

Hydrobromic  in  the  Presence  of  Hydriodic  Acid. 

Procedure.  —  Add  to  the  neutral  or  slightly  acid  solution 
CS2,  and  then  chlorine  water  little  by  little,  with  constant 
shaking,  until  the  violet  color  of  the  iodine  disappears. 

Note.  —  All  the  iodine  is  set  free  before  any  of  the  bromine. 
Excess  of  chlorine  oxidizes  it  to  iodic  acid,  which  is  colorless : 
the  bromine  is  then  set  free  and  gives  a  red  or  yellow  color 
to  the  CS2.  Excess  of  chlorine  also  decolorizes  it,  as  above 
stated  ;  hence  the  need  of  adding  the  reagent  very  slowly. 

Hydrochloric  in  the  Presence  of  Hydrob^omic  and  Hydriodic 

Acids. 

Procedure.  —  Mix  the  finely  powdered  substance  intimately 
with  dry  powdered  K2Cr2O7 ;  place  in  a  small  dry  distilling 


NITRIC   ACID   GROUP.  6r 

flask,  cover  with  concentrated  H2SO4,  and  heat.  Cause  the 
vapors  to  pass  into  a  test  tube  containing  a  little  dilute 
NH4OH. 

Notes.  —  i.  When  chlorides  are  present,  deep  red  vapors 
of  chromium  oxychloride,  CrO2Cl2,  are  evolved.  The  corre- 
sponding bromine  compound  is  not  formed,  but  free  bromine 
escapes.  The  reactions  in  the  two  cases  are  : 

K2Cr2O7  +  4NaCl  -f-  3H2SO4  =  2CrO2Cl2  +  K2SO4  -f 

2Na2SO4  +  3H2O. 
K2Cr207  +  6KBr  +  7H2SO4  =  3Br2  +  Cr2(SO4)3  + 

4K2SO4  +  7H2O. 

2.  NH4OH  reacts  with  CrO2Cl2  with  formation  of  NH4C1 
and  (NH4)2CrO4,  which  imparts  a  yellow  color  to  the  liquid. 
With  Br,  NH4Br  and  NH4BrO3  are  formed,  both  of  which  are 
colorless. 

3.  H2O  decomposes  CrO2Cl2 ;  hence  the  need  of  having 
the  apparatus  and  the  salts  perfectly  dry. 

4.  Care  must  be  taken  that  none  of  the  K2Cr2O7  is  carried 
over  mechanically  by  frothing   or   spurting ;   for  this  would 
of  course  give  rise  to  a  yellow  coloration  on  the  addition  of 
NH4OH. 

5.  The  presence  of  much  HI  interferes  with  this  test  for 
HC1.     It  is  removed  by  precipitating  with  AgNO3,  and  treat- 
ing the  precipitate  with  strong  NH4OH,  which  dissolves  AgCl 
readily  and  Agl  only  very  slightly.    The  AgCl  is  reprecipitated 
by  HNO3  and  boiled  or  fused  with  Na2CO3 ;  the  solution  is 
filtered  and  evaporated,  and  the  test  with  K2Cr2O7  and  H2SO4 
made  in  the  usual  way. 

NITRIC   ACID    GROUP. 

Nitric  Acid. 

Procedure.  —  Mix  the  solution  to  be  tested  with  an  equal 
volume  of  strong  H2SO4,  cool,  and  pour  a  concentrated 
FeSO4  solution  cautiously  down  the  side  of  the  tube,  so 
that  the  two  liquids  do  not  mix  ;  allow  it  to  stand  some 
minutes,  if  no  color  appears. 


62  DETECTION  OF   THE   ACIDS. 

Note.  —  The  H2SO4  sets  free  HNO3  from  the  nitrate;  the 
HNO3  is  then  reduced  to  NO  by  the  FeSO4,  and  the  brown 
ring  is  due  to  the  formation  of  a  compound  of  the  two  sub- 
stances last  named.  This  compound  is  decomposed  by  heat. 

Procedure.  —  Treat  a  small  quantity  of  the  dry  substance 
with  a  few  drops  of  phenolsulphonic  acid,  dilute  with  water, 
and  make  alkaline  with  NH4OH. 

Notes.  —  i.  Picric  acid  is  produced  by  the  action  of  HNO3 
on  the  phenol,  and  its  formation  is  indicated  by  the  intensely 
yellow  color  of  its  ammonium  salt. 

2.  The  test  is  exceedingly  delicate,  and  serves  to  detect 
very  minute  quantities  of  HNO3.  It  is  used,  for  example,  in 
testing  natural  waters,  these  being  first  evaporated  to  dryness. 

Chloric  Acid. 

Procedure.  —  Add  strong  H2SO4  to  a  little  of  the  solid 
substance  on  a  watch  glass,  and  heat  gently. 

Note.  —  C1O2,  a  greenish   yellow  gas  smelling  like  Ci,  is 
evolved,  which  colors  the  H2SO4  intensely  yellow.     The  re- 
action is  : 
3KC1O3  +  2H2SO4  =  2KHSO4  +  KC1O4  +  2C1O2  +  H2O. 

Procedure.  —  Ignite  the  solid  substance  in  a  porcelain  dish 
at  a  low  red  heat.  Dissolve  in  water  and  add  AgNO3. 

Note.  —  This  test  is  very  delicate,  but  is  applicable,  of 
course,  only  in  the  absence  or  after  the  removal  of  chlorides 

Nitric  in  the  Presence  of  Chloric  Acid. 

Procedure.  —  Mix  the  substance  in  a  small  porcelain  dish 
with  dry  Na2CO3  and  ignite  gently  for  five  to  ten  minutes. 
Test  in  the  usual  way  for  HNO3. 

Notes.  —  i.  The  presence  of  HC1O3  prevents  the  reduction 
of  HNO3  in  the  FeSO4  test,  and  gives  a  brown  color  to  the 
phenol  sulphonic  acid ;  it  must  therefore  be  removed. 


ORGANIC   ACIDS.  63 

2.  The  chlorate  is  converted  to  chloride  by  ignition  ;  the 
nitrate    is   unaffected  or  partially   reduced   to   nitrite,  which 
gives  the  same  reaction  with  FeSO4.     The  use  of  too  high 
a  temperature  would  destroy  the  nitrate  and  nitrite.  . 

3.  Nitrates    except    those   of   the   alkali-metals   are   very 
easily  decomposed  by  heat  ;   hence  the  addition  of 


Nitric  in  the  Presence  )f  Chromic  Acid. 

Procedure.  —  Warm  the  solution  with  addition  of  H2SO3 
till  the  color  becomes  a  pure  green.  Add  NH4OH  and  fil- 
ter ;  test  the  filtrate  with  H2SO4  and  FeSO4. 

Nitric  in  the  Presence  of  Hydriodic  Acid. 

Procedure.  —  Add  Ag2SO4  to  the  solution  acidified  with 
H2SO4,  as  long  as  a  precipitate  continues  to  form  ;  filter  and 
test  the  filtrate  for  HNO3. 

ORGANIC    ACIDS. 
Acetic  Acid. 

Procedure.  —  Add  a  mixture  of  equal  volumes  of  alcohol 
and  strong  H2SO4  to  the  solid  substance  placed  in  a  test 
tube  ;  heat  gently,  and  _iote  the  odor. 

Notes.  —  i.  The  alcohol  and  acetic  acid  react  in  the  pres- 
ence of  H2SO4  with  formation  of  ethyl  acetate,  C2H5(C2H3O2), 
a  volatile  liquid  with  a-  pleasant  ethereal  odor. 

2.  If    the  mixture  is  heated  too  hot,  other  products  (ether, 
SOo,  etc.)  are  formed  by  the  action  of  the  H2SO4  on  the 
C2H5OH,  whereby  the  odor  of  the  ethyl  acetate  is  concealed. 

3.  Till   perfectly  familiar  with  the  odor,  the   student  in 
trying  the  test  should  always  make  a  comparative  experiment 
with  a  pure  acetate. 

Tartaric  Acid. 

Procedure.  —  Evaporate  the  solution  to  the  volume  of  a 
few  drops,  slightly  acidify  with  HC2H3O2,  add  some  KC2H3Oj 
solution,  stir  with  a  glass  rod,  and  allow  it  to  stand. 


64  DETECTION  OF    THE  ACIDS. 

Notes.  —  i.  The  crystalline  precipitate  is  KHC4H4O6,  po- 
tassium acid  tartrate,  cream  of  tartar.  The  test  is  exceedingly 
characteristic,  for  no  other  potassium  salt  (except  K2PtCl6) 
is  nearly  so  insoluble.  It  is  not  very  delicate,  however,  owing 
to  the  slight  solubility  of  the  precipitate  in  water ;  still,  if  care 
is  taken  to  have  the  solution  very  concentrated,  and  if  time  is 
allowed,  it  is  sufficiently  so  for  most  purposes. 

2.  Addition  of  C2H5OH  greatly  diminishes  the  solubility 
of  the  precipitate;  before  adding  it,  however,  it  is  necessary  to 
make  sure  that  it  alone  (without  KC2H3O2)  does  not  give  a 
precipitate  with  the  solution. 

3.  If  the  solution  contains  HC1  or  HNO3,  this  should  be 
removed  by  evaporating  to  dryness  on  the  water  bath,  since 
strong  acids  readily  dissolve  KHC4H4O6. 

4.  If  the  solution  contains  potassium  salts,  the  precipitate 
forms,  of  course,  as  soon  as  the  HC2H3O2  is  added.     In  pre- 
paring the  solution  to  test  for  tartrates,  it  is  better  to  boil  the 
substance,  if  insoluble,  with  K2CO3  than  with  Na2CO3  solu- 
tion ;  for  potassium  salts  diminish  and  sodium  salts  increase 
the  solubility  of  KHC4H4O6. 

Procedure.  —  Make  the  solution  slightly  alkaline  witlr 
NH4OH,  add  CaCl2  in  excess,  allow  it  to  stand  a  short  rime, 
filter,  treat  the  precipitate  in  the  cold  with  strong  NaOH, 
dilute,  filter  if  not  clear,  and  heat  the  nitrate  to  boiling. 

Note.  —  CaC4H4O6  is  precipitated  from  neutral  or  ammo- 
niacal  solution  by  CaCl2.  Presence  of  ammonium  salts  re- 
tards the  precipitation.  The  solubility  of  the  precipitate  in 
cold  NaOH  and  its  reprecipitation  on  boiling  are  pauicularty 
characteristic  of  tartaric  acid 


ANALYSIS    IN    THE    DRY    WAY. 


GENERAL    REMARKS. 

The  methods  so  far  described  make  it  possible  to  detect 
and  separate  the  metals  and  acids  only  when  in  solutior. .  Valu- 
able information  in  regard  to  the  composition  of  a  material  may, 
however,  often  be  obtained  by  making  a  few  simple  tests  directly 
with  the  solid  substance.  Such  tests  are  of  value,  first,  as  a  pre- 
liminary indication  of  the  presence  of  specific  elements  or  com- 
pounds ;  for  the  knowledge  thus  obtained  often  facilitates  the  pre- 
paration of  the  solution  for  analysis,  and  enables  the  subsequent 
analysis  in  the  wet  way  to  be  more  intelligently  carried  out.  They 
are  of  value,  secondly,  as  an  independent  method  of  analysis 
which  may  replace  the  more  lengthy  wet  process  in  cases  where 
information  only  in  regard  to  the  presence  or  absence  of  particular 
elements  is  desired,  or  where  the  facilities  of  a  well-equipped  lab- 
oratory are  not  available. 

Where  a  complete  analysis  in  the  wet  way  is  to  be  made, 
it  is  usually  not  worth  while  to  make  all  the  tests  described 
below ;  for  the  physical  properties  of  the  substance,  its  behavior 
towards  solvents,  and  the  precipitates  produced  by  the  general 
reagents,  indicate  in  most  cases  very  quickly  the  nature  of  the 
metals  present  in  considerable  proportion.  The  closed  tube  test 
should,  however,  always  be  made  ;  for  it  shows  at  once  the  pres- 
ence or  absence  of  organic  matter  and  of  water,  and  in  many 
ca^es  the  nature  of  the  acids  contained  in  the  substance.  More- 
over, if  any  special  difficulty  is  met  with  in  getting  the  substance 
into  solution,  it  is  always  advisable  to  try  all  the  dry  tests  before 
proceeding. 

It  should  be  understood,  however,  that  these  dry  tests, 
especially  when  applied  to  complex  substances  with  negative  results 


66  ANALYSIS    IN    THE    DRY    WAY. 

are  not  as  delicate  nor  conclusive  as  those  made  in  the  wet  way  ; 
also  that  some  of  the  reactions  are  readily  obtained  and  correctly 
interpreted  only  after  considerable  experience.  The  student, 
therefore,  should  not  at  first  draw  too  positive  conclusions  from 
the  results 

SPECIAL    TESTS. 

Procedure.—  Place  a  little  of  the  finely  powdered  substance 
in  a  glass  tube  sealed  at  one  end.  Heat  gently  at  first,  and 
then  to  the  highest  heat  of  the  flame.  Note  any  change  of 
appearance  and  any  odor.  During  the  heating  apply  a  flame 
to  the  mouth  of  the  tube,  and  insert  in  it  a  glowing  wood- 
splinter  or  a  smouldering  bit  of  twine. 

Notes. —  i.  The  important  indications  furnished  by  this  test 
are  in  regard  to  the  presence  of  (i)  organic  matter,  (2)  water, 
(3)  compounds  of  volatile  metals  and  (4)  in  regard  to  the 
nature  of  the  acid  elements  of  the  substance.  The  character- 
istic phenomena  that  may  be  observed  are  (i)  carbonization, 
(2)  formation  of  a  distillate  or  sublimate,  (3)  evolution  of  a 
gas,  (4)  fusion  or  change  of  color. 

2.  Especial  attention  must  be  given  to  determining  whether 
the  substance  carbonizes  ;  first,  because,  if  organic  matter  is 
present  it  must  be  destroyed  (as  described  on  page  74),  before 
proceeding  with  the  detection  of  the  metals  ;  and  secondly, 
because,  if  organic  matter  is  not  present,  the  tests  for  organic 
acids  may  be  omitted.     For  a  fuller  discussion  of  carboniza- 
tion see  the  notes  on  page  51. 

3.  As  water  is  used  in  preparing  the  solution,  its  presence 
must  of  course  be  determined  by  a  test  made  with  the  original 
substance.     If  present,  it  should  always  be  reported.    In  order 
to  detect  it,  keep  the  upper  part  of  the  tube  as  cool  as  possi- 
ble during  the  first  part  of  the  heating,  so  as  to  cause  it  to 
condense.     It  may  be  present  in  the  substance  as  water  of 
constitution,  as  in  FeO3H3  and  Na2HPO4 ;  as  water  of  crys- 
tallization, as  in  MgSO4.7H2O;  as  hygroscopic  moisture  on 
the  surface  ;    and  as  water  mechanically  enclosed  within  the 
crystals.   Speaking  generally,  the  temperature  required  for  the 
expulsion  of  water  is  lower  in  the  order  named. 


ANALYSIS  IN   THE  DRY  WAY. 


67 


4.     The  sublimates  which  may  be  obtained  and  the  corre- 
sponding indications  are  : 


Black,  accompanied  by  a  garlic  odor      .     . 
Black,  forming  minute  globules  when  rubbed 
Black,  becoming  red  when  rubbed     .     .     , 
Black,  accompanied  by  violet  vapors  .     »     . 

Reddish  brown  drops  becoming  / 

a  yellow  solid  on  cooling          \      '     '     * 


As 

Hg  or  a  compound  of  it. 

HgS. 

I. 


.     .     .     S  or  a  persulphide. 
Yellow S  or  As2S3. 

White,  quite  volatile  ,     f  An  NH4  salt,  As2O3,  HgCl, 

{    HgCl2,  or  an  organic  acid. 

White,  difficultly  volatile Sb2O3. 

From  all  compounds  of  NH4,  Hg,  and  As  (except  As2O5, 
arsenites,  and  arseniates)  these  components  volatilize  read- 
ily and  completely.  Therefore,  if  the  substance  containing 
them  was  for  the  most  part  insoluble,  and  was  decomposed 
by  fusion,  they  might  escape  detection  altogether,  if  the 
closed  tube  test  were  omitted. 

5.  The  gases  that  may  be  evolved  and  the  compounds 
that  they  indicate  are  shown  in  the  following  table : 


Gas: 


Known  by: 


Indicates : 


02 
N02 
SO2 

NH3 

H2S 

CO 

CO2 

C12;  Br2;  I2 


Inflaming  a  glowing  splinter. 

Its   odor,  and  brownish-red  J 
color.  ) 

Its  odor. 

Its  odor. 

(  Its  odor  and  by  blackening  f 
}      lead  acetate  paper.  J 

Burning  with  a  blue  flame. 
(  Causing  turbidity  in  a  drop  ) 
]      of  BaO2H2.  J 

Their  odor  and  color. 


A  chlorate,    peroxide,  or  al- 
kali  nitrate. 

A  nitrate  or  nitrite. 

A  sulphate,  sulphite,  or  sul-  ( 
phide.  \ 

An  ammonium  salt,  a  cyan-  ) 
ide  or  nitrogenous  organic  > 
matter.  ) 

A  moist  sulphide. 

An  oxalate. 

A  carbonate,  oxalate,  or  or-  ) 

ganic  matter.  I 

A  chloride,  bromide,  or  iodide 


ANALYSIS  IN   THE  DRY  WAY. 


6.     The  substance  may  undergo  a  change  of  color.     Some 
of  the  indications  are  as  follows : 


Original  color  : 

Color  while  hot: 

Color  after  cooling  : 

Indication  : 

White. 

Yellow. 

White. 

Zn. 

White. 

•    Yellow. 

Pale  yellow. 

Sn. 

White  or  yellow. 

Brownish  red. 

Deep  yellow. 

Pb. 

White  or  yellow. 

Brownish  red. 

Pale  yellow. 

Bi. 

Yellow  or  brown. 

Black. 

Brownish  red. 

Fe. 

Yellow  or  red. 

Green. 

Green. 

A  chr  ornate. 

Pink,  green,  or  blue. 

Black. 

Black. 

Co,  Ni,  Cu. 

7.  If  the  substance  fuses  without  volatilization  or  expulsion 
of  aqueous  vapors,  or  if  it  remains  in  a  state  of  fusion  after  vola- 
tile matters  have  been  expelled,  it  is  probably  a  compound  of  an 
alkali  metal.  In  that  case  try  the  color  imparted  by  it  to  the 
flame. 

Procedure.  —  Heat  some  of  the  dry  powder  in  a  small 
cavity  on  a  piece  of  charcoal  before  the  reducing  flame  of 
the  blowpipe.  Observe  the  odor  evolved  and  the  nature 
of  the  incrustation  and  of  the  residue  in  the  cavity. 

JV0tes.  —  i.  The  metals  reduced  to  the  metallic  state  and 
forming  globules  are  Pb,  Ag,  Bi,  Sb,  Cu,  and  Sn.  Bi  and  Sb 
are  distinguished  by  their  brittleness  from  the  others ;  Cu  is 
known  by  its  red  color.  (Au  forms  a  yellow  globule.) 

2.  The  characteristic  incrustations  formed  are  the  following  : 
White,  very  volatile,  at  a  distance  from  the  cavity :  As. 
White,  volatile  with  some  difficulty :  Sb. 

Yellow  when  hot,  white  when  cold  :  Zn  and  Sn. 
Yellow  both  hot  and  cold :  Pb  and  Bi. 
Reddish  brown  :  Cd. 
Dark  red :  Ag. 

3.  The   evolution   of  a  garlic  odor  indicates  As,  and   of 
SO2,  a  sulphide.     If  the  substance  deflagrates,  it  is  probably 
a  chlorate  or  nitrate. 


ANALYSIS  IN    THE    DRY    WAY. 


69 


Procedure.  —  Make  a  clear  bead  by  fusing  some  NaPO3 
or  (NH4)NaHPO4  on  the  loop  of  a  platinum  wire,  add  to 
it  a  little  of  the  substance  to  be  tested,  and  heat  it  in  the 
oxidizing  flame  of  a  Bunsen  burner.  Observe  the  color  of 
the  bead  when  both  hot  and  cold.  Then  heat  it  in  the 
reducing  flame. 

Notes. —  i.  The  color  imparted  to  the  bead  depends  in 
many  cases  on  whether  it  is  heated  in  the  oxidizing  or  re- 
ducing flame.  To  obtain  an  oxidizing  action,  heat  the  bead 
in  the  edge  or  at  the  top  of  the  non-luminous  flame  of  the 
Bunsen  burner.  To  get  a  reducing  action,  cut  off  the  supply 
of  air  to  the  burner  enough  to  produce  a  small  luminous  cone 
in  the  interior  of  the  flame,  and  heat  the  bead  in  this  cone. 

2.  The  following  are  the  characteristic  colorations  which 
the  cold  bead  may  exhibit :  Blue  in  both  flames  :  Co.     Green 
and  in  both  flames  :   Cr.     Greenish  blue  in  the  oxidizing,  red 
and  opaque  in  the  reducing  flame  :  Cu.     Amethyst  red  in  oxi- 
dizing, colorless  in  the  reducing  flame  :  Mn.     Brownish  red 
in   the  oxidizing  flame  while   hot,  yellow  or  colorless  when 
cold  :  Fe  or  Ni.     Enamel-white  after  cooling  :  Ba  or  Sr  ;  but 
many  other  metals  produce  this  same  effect,  when  a  consider- 
able amount  of  the  substance  containing  them  is   added  to 
the  bead. 

3.  One    of   the   most  important  indications  of   the   bead 
test  is  in  regard  to  silicic  acid.      If  this  substance   is  pres- 
ent, it  floats  about  undissolved  in  the  melted  bead  as  a  semi- 
transparent  skeleton.     This  distinguishes  it  from  all  metallic 
oxides. 

4.  By  the  action  of  heat  NH4NaHPO4  changes  to  NaPO3, 
sodium  metaphosphate  ;    and    it  is   far   more    convenient    in 
practice  to  make  the  beads  directly  with  the  latter  substance, 
which  can  be  prepared  from  the  former  in  quantity  by  igni 
tion.      The  colors  imparted  by  different  metals   are   due  to 
the    formation    of    double    orthophosphates ;    for    example, 
MnNaPO4,  CoNaPO4,  etc. 

5.  Borax,  Na2B4O7,  is  sometimes  used  in  place  of  NaPO3. 
The  colors  produced    are    the    same  in  most  cases,   but  not 
in  all.     It  is,  however,  in  general  less  suitable  than  NaPO8; 
for  it  does  not  show  satisfactorily  the  silicic  acid  reaction. 


7o 


ANALYSIS  IN   THE  DRY  WAY. 


Procedure.  —  Add  a  few  drops  of  strong  H2SO4  to  some 
of  the  finely  powdered  substance  placed  in  a  very  small  test- 
tube,  and  heat  gently  —  enough  to  cause  volatilization  of 
the  H2SO4. 

Note.  —  For  the  indications  furnished  by  this  test,  see  the 
notes  on  page  54.  It  is  valuable  especially  as  a  means  of 
detecting  certain  Volatile  acids  which  may  not  be  shown  by 
the  closed-tube  test. 

Procedure.  —  Introduce  on  the  loop  of  a  clean  platinum 
wire  a  little  of  the  substance  moistened  with  H2SO4  into 
the  outer  edge  of  the  lower  part  of  the  Bunsen  flame.  As 
soon  as  the  coloration  has  nearly  ceased,  moisten  the  sub- 
stance on  the  wire  with  HC1,  and  again  introduce  it  into 
the  flame.  Observe  the  flame  closely  and  continuously 
until  the  substance  is  burnt  off.  If  it  is  colored  yellow, 
look  at  it  through  a  thick  piece  of  blue  cobalt  glass. 

[Examine  the  flame  by  means  of  a  spectroscope,  and  com- 
pare the  spectrum  as  to  the  position  of  its  lines  with  the 
spectra  produced  by  pure  compounds  of  the  elements  sus- 
pected to  be  present.] 

Notes.  —  i.  The  characteristic  flame  colorations  are  as 
follows : 

Yellow:  Na. 

Red  :  Ca,  Sr,  (Li). 

Violet  :  K,  (Rb,  Cs). 

Green  :  Ba,  Cu,  H3BO3,  (Tl). 

2.  By  proceeding  as  here  directed  elements  may  often  be 
detected  even  in  the  presence  of  one  another  by  their  flame 
colorations  ;  for  the  alkali  metals  first  volatilize,  leaving  be- 
hind the  alkaline-earth  sulphates,  which  can  then  be  detected 
by  moistening  with  HC1,  since  their  chlorides  are  much  more 
volatile  than  their  sulphates.  '  Moreover,  the  use  of  blue  glass 
makes  it  possible  to  detect  potassium  in  the  presence  of  so- 
dium, for  the  yellow  light  caused  by  the  latter  metal  is  thus 
completely  absorbed.  It  should  always  be  proved,  however, 
by  testing  with  a  pure  sodium  salt,  that  the  blue  glass  is  of 
sufficient  thickness  to  cut  off  the  sodium  light  completely. 


ANALYSIS  JN   THE   DRY   WAY.  yx 

3.  The  spectroscope  is  a  valuable  aid  in  qualitative  an- 
alysis as  a  means  of  detecting  elements  present  in  such 
minute  quantities  that  they  are  not  revealed  by  the  ordi- 
nary methods,  especially  in  testing  the  purity  of  a  sub- 
stance, in  examining  mineral  waters,  or  any  other  material 
of  which  only  a  small  amount  is  available,  in  detecting  the 
rarer  elements  in  minerals,  etc.  It  may  also,  in  special 
cases,  be  used  with  advantage  as  a  substitute  for  the  far 
more  lengthy  wet  processes ;  for  example,  in  the  analysis 
of  the  alkaline-earth  group,  or  of  the  group  of  rare  earthy 
metals :  it  is  not,  however,  usually  a  satisfactory  substitute 
for  these ;  for  a  good  qualitative  analysis  should  not  only 
establish  the  presence  or  absence  of  the  elements,  but  also 
determine  roughly  the  relative  proportions  of  those  present, 
a  thing  which  spectrum  analysis  (in  its  ordinary  form)  en- 
tirely fails  to  do.  —  The  elements,  which  give  the  most  char- 
acteristic spectra  when  heated  in  the  flame  are  Na,  K,  Li, 
Rb,  Cs,  Ba,  Sr,  Ca,  Tl,  In. 


PREPARATION    OF  THE    SOLUTION. 


I.     FOR    THE    ANALYSIS    FOR    METALS. 
Non-Metallic  Substances. 

Procedure.  —  Place  about  one  gram  of  the  finely  powdered 
substance  in  a  test  tube,  add  considerable  water,  and  heat 
to  boiling.  If  the  substance  dissolves  completely,  test  the, 
solution  with  litmus  paper,  and  use  it  for  the  analysis  for 
metals.  If  in  doubt  whether  anything  dissolves,  filter  a 
little  of  the  liquid,  and  cautiously  evaporate  three  or  four 
drops  to  dryness  on  a  small  watch  glass.  If  partial  solution 
takes  place,  allow  the  substance  to  settle  out,  decant,  and 
boil  the  residue  with  a  fresh  portion  of  water.  If  the  sub- 
stance does  not  now  dissolve,  add  to  the  water  a  few  drops 
of  HC1,  and  heat,  noting  carefully  whether  effervescence  or 
evolution  of  any  odor  occurs  ;  if  necessary,  add,  little  by 
little,  more  HC1.  In  case  dilute  HC1  does  not  effect  solu- 
tion, rinse  the  residue  into  a  porcelain  dish,  decant  the 
liquid,  and  boil  for  some  time  with  strong  HC1.  If  this  does 
not  suffice,  add  one  third  the  volume  of  strong  HNO3,  and 
heat  again.  A  residue  remaining  after  continued  action  of 
aqua  regia  must  be  fused  with  Na^Og.  Mix  together  the 
various  liquids  by  which  any  of  the  substance  has  been  dis- 
solved. If  only  dilute  HC1  has  had  to  be  used,  dilute  the 
solution  and  pass  in  H2S  directly.  If  much  strong  HC1  or 
any  HNO3  was  necessary,  evaporate  the  solution  just  to 
dryness,  moisten  the  residue  with  a  few  drops  of  HC1,  add 
considerable  water,  heat,  and  pass  in  H2S.  For  modifica- 
tions of  this  procedure  in  certain  special  cases,  see  the  fol- 
lowing notes  4,  5,  and  6. 


FOR    THE  ANALYSIS  FOR  METALS.  73 

i.  An  acid  reaction  of  the  aqueous  solution  may 
be  due  to  free  acids,  acid  salts  of  strong  acids,  or  to  neutral 
salts  of  most  metals  of  the  H2S  and  (NH4)2S  groups.  An  alka- 
line reaction  shows  the  presence  of  a  soluble  hydrate,  carbon- 
ate, sulphide,  phosphate,  borate,  or  cyanide.  It  is  to  be  noted 
that  the  terms  "acid  salt"  and  "neutral  salt"  refer  only  to  the 
extent  of  the  replacement  of  the  hydrogen  of  the  acid  by 
the  metal,  and  do  not  show  the  reaction  towards  litmus  or 
other  indicators.  The  latter  depends  on  the  relative  strength 
of  the  acid  and  base.  Examples  :  Na2CO3,  Na2S,  and  even 
the  acid  salts,  NaHCO3,  Na2HPO4,  react  alkaline.  ZaSQ*, 
CuCl2,  etc.,  react  acid. 

2.  It  is  desirable  to  use  as  little  HC1  as  possible  in  dis- 
solving the  substance,  since  otherwise  it  is  necessary  to  evap- 
orate the  solution  before  passing  in  H2S. 

3.  HC1  may  cause  evolution  of  CO2,  H2S,  SO2,  HCN,  or 
Cl  (from  chlorates,  chromates,  or  peroxides),  thus  indicating 
the  nature  of  the  acid  present. 

4.  HC1  may  also  cause  separation  of  gelatinous  silicic  acid. 
When  this  occurs,  and  even  when    it  does  not,  if   the  sub- 
stance is  a  mineral  or  metallurgical  product  liable  to  contain 
silicate,  the  solution  should  be  evaporated  to  dryness,  and  the 
residue  ignited  at  125°  in  the  hot  closet  for  at  least  an  hour; 
the  mass   should  then  be    dissolved  in   a  little  HC1,  water 
added,  the  solution  filtered,  and  the  analysis  proceeded  with. 
This  is  done  to  remove  the  silicic  acid,  which  in  the  hydrated 
condition  dissolves  to  a  considerable  extent  in  acid,  but  which 
is  dehydrated  and  made  insoluble  by  ignition.     This  could, 
of  course,  be  accomplished  by  direct  ignition  over  the  lamp ; 
but  it  is  then  very  difficult  to  get  A12O3  and  Fe2O3  again  into 
solution.     If  the  silicic  acid  is  not  removed,  it  precipitates  on 
the  addition  of  NH4OH,  and  may  be  mistaken  for  A1O3H3. 

5.  In  case  the  preliminary  examination  indicates  the  pres- 
ence of  lead  or  silver,  treat  the  substance  first  with  HNOS 
instead  of  with  HC1;  then  decant  and  use  aqua  regia  if  neces- 
sary.    Or  if  in  any  case  HC1  has  been  tried,  and  has  been 
found  to  leave   a  white  residue  resembling  PbCl2  or  AgCl, 
treat  a  fresh  portion  of  the  substance  with  HNO3. 

6.  If  the  preliminary  examination  shows  the  presence  of 
organic   matter,   this    must  be  destroyed  before    proceeding, 


74  PREPARATION  OF   THE  SOLUTION 

since  it  interferes  with  the  various  tests,  particularly  with 
the  precipitation  of  the  trivalent  metals  by  NH4OH.  The 
methods  used  for  destroying  it  vary  with  the  nature  of  the 
substance,  and  directions  cannot  be  given  to  cover  every  case. 

a.  If  the  proportion  of  mineral  matter  is  large,  ignite  the 
substance  at  a  low  temperature    in  a  porcelain    crucible  till 
fully  charred ;    cool1,  moisten  with   HNO3,    and    ignite    again 
carefully  at  first,  and  then  strongly.     Repeat  the  addition  of 
HNO3  and   ignition   till    the  carbon  is  consumed.     Boil  the 
residue  with  a  small  quantity  of  strong  HC1,   dilute,  filter, 
and  proceed  with  the  analysis  as  usual.    This  method  is  appli- 
cable, for  example,  to  baking  powders,  which  it  is  often  neces- 
sary to  test  for  Al.     Mercury,  ammonium,  arsenious  and  anti- 
monious  compounds  are  volatilized. 

b.  If  the  substance  is  mostly  organic,  heat  it  in  a  porcelain 
dish  on  the  steam  bath  for  ten  or  fifteen  minutes  with  a  mix- 
ture of  about  equal  parts  of  strong  H2SO4  and  HNO3,  then 
heat  over  a  lamp  till  white  fumes  of  H2SO4  escape ;  cool,  add 
HNO3  and  heat  again  •   repeat  this  process  till  the   H2SO4 
becomes  light  colored.     Dilute  with  water,  boil  to  expel  SO2 
and  filter.     This  method  is  suitable  for  destroying  paper,  fab- 
rics, foods,  etc.     The  H2SO4  solution  so  obtained  may  be  put 
directly  into  a  hydrogen  generator  and  tested  for  As  and  Sb 
(page  25)  ;  for  As  and  Sb  in  the  higher  form  of  oxidation,  to 
which  they  are  converted  by  the  HNO3,  are  not  volatile.     In 
using  this  method  bear  in  mind  the  insolubility  of  the  sulphates 
of  Ba,  Sr,  Ca,  and  Pb. 

c.  If  the   substance   contains  oily  or  fatty  matter,  extract 
this  by  treating  the  substance  two  or  three  times  with  ether, 
and  use  the  residue  for  analysis.     Paints  in  oil,  for  example, 
are  conveniently  treated  in  this  way. 

7.  If  sulphides  are  present,  and  HNO3  or  aqua  regia  is  used 
for  dissolving,  sulphur  separates  in  the  form  of  a  light  spongy 
mass,  often  black  at  first  owing  to  inclosed  matter,  but  becom- 
ing yellow  on  continued  boiling.     It  may  be  tested  by  heating 
on  porcelain  when  it  burns  with  a  blue  flame  and  odor  of  SO2. 

8.  When  strong  acids  have  been  used  for  dissolving  the 
substance,   on  evaporating   these    a  white    residue    insoluble 
in  dilute  HC1  is  sometimes  obtained.      This  may  be  SiO2, 
PbSO4,  CaSO4,  CaF2,  or  H2SnO3.     If  there  is  reason  to  think 
it  may  be  PbSO4,  treat  it  with  (NH4)C2H3O2  solution.     (See 


FOR    THE  ANALYSIS  FOR  METALS.  75 

note  2,  <r,  page  76).  Test  it  for  CaSO4  or  CaF2,  by  trying 
the  color  it  imparts  to  the  flame  when  moistened  with  HCI. 
Test  it  for  Sn  before  the  blowpipe  on  charcoal. 

9.  If  gelatinous   silicic   acid   separates    on    treating   with 
HCI,  it  is  impossible  to  tell  whether  the  substance  has  been 
completely  decomposed.     To  determine  this,  filter,  wash  the 
precipitate,  and  boil  it  with  Na2CO3.     If  it  is  pure  silicic  acid, 
it  will  entirely  dissolve. 

10.  It  is  not  always  advisable  to  mix  the  aqueous  and  acid 
solutions  of  the  substance  as  has  been  directed ;  for  they  may 
precipitate  each  other.     For  example,  in  treating  in  the  usual 
way  a  substance  composed  of  Na2SO4  and  BaCO3,  the  former 
salt  would  dissolve  in  water,  and  the  latter  in  dilute  HCI,  but 
on  mixing  the  two  solutions  insoluble  BaSO4  would  precipi- 
tate.    In  practice,  therefore,  always  mix  small  portions  of  the 
two  solutions  first.     Even  where  they  do  not  precipitate  each 
other,  they  are  sometimes  analyzed  separately,  in  order  to  learn 
how  the  metallic  and  acid  radicals  are  combined  with  each  other. 

11.  Substances  soluble  only  in  hot  water  or  concentrated 
acids,  and  separating  out  on  cooling  or  dilution   are  to  be 
treated  as  insoluble. 

Procedure.  —  If  an  insoluble  residue  remains  after  the 
treatment  with  acids,  wasb  it  thoroughly  and  try  such  of 
the  tests  and  separations  described  in  note  2  below  as  the 
appearance  of  the  residue  and  previous  indications  warrant. 
If  a  residue  still  remains,  dry  it,  mix  it  with  five  or  six  times 
its  weight  of  Na2CO3,  and  fuse  it,  in  platinum  if  reducible 
metals  are  known  to  be  absent  by  the  preliminary  examina- 
tion on  charcoal  or  by  the  tests  just  tried,  otherwise  in  a 
covered  porcelain  crucible.  If  acids  have  not  acted  on  the 
substance,  or  if  they  have  dissolved  only  a  small  part  of  it, 
fuse  a  very  finely  powdered  portion  of  the  original  substance 
instead  of  that  treated  with  acids.  Use  enough  Na2CO3  and 
a  high  enough  heat  to  make  the  mixture  fuse  to  a  thin  liquid ; 
continue  the  heating  ten  or  fifteen  minutes,  or  longer  if  effer- 
vescence has  not  ceased.  Boil  the  fused  mass  with  water 
till  disintegrated,  filter  and  wash  the  residue  till  free  from 
Dissolve  a  small  part  of  it  in  dilute  HNO3,  and 


7  6  PREPARATION   OF    THE    SOLUTION 

test  the  solution  with  HC1  for  Ag  (and  Pb) ;  dissolve  the 
remainder  in  dilute  HC1.  Mix  with  a  small  portion  of  this 
solution  a  little  of  the  aqueous  solution  of  the  fusion,  and 
make  the  mixture  acid,  if  it  is  not  already  so.  If  no  precipi- 
tate forms,  reserve  one  half  of  the  aqueous  solution  for  the 
tests  for  acids,  and  mix  the  other 'half  with  the  acid  solution 
of  the  fusion ;  acidify  if  alkaline,  evaporate  to  dryness,  and 
ignite  for  an  hour  or  more  at  125°  to  render  silica  insoluble  ; 
dissolve  in  a  little  HC1,  dilute  with  water,  filter,  and  proceed 
with  the  analysis  (page  17).  If  the  aqueous  and  acid  solu- 
tions precipitate  each  other  on  mixing,  use  only  the  acid 
solution  for  the  analysis  for  metals.  For  modifications  of 
the  method  of  fluxing  applicable  in  special  cases,  see  notes 
7  and  8  below. 

Notes.  —  i.  The  important  substances  insoluble  or  diffi- 
cultly soluble  in  water  and  acids  which  may  be  found  in  the 
insoluble  residue  are  :  C,  S,  BaSO4,  SrSO4,  CaSO4,  PbSO4, 
PbCl2,  AgCl,  CaF2,  silica  and  many  silicates,  and  the  native 
or  ignited  oxides  of  Al,  Cr,  Fe,  and  Sn. 

2.  It  is  often  advantageous  to  try  special  tests  for  some  of 
these  substances  and  to  remove  them  if  present  before  fusing ; 
first,  because  fusion  may  thereby  become  unnecessary,  and 
secondly,  because  it  can  then  almost  always  be  made  in  plati- 
num vessels.  These  tests,  which  should  be  tried  when  there 
are  indications  that  the  substances  tested  for  are  present,  are 
as  follows  : 

a.  Heat  the  residue  in  a  porcelain  crucible  with  free  access 
of  air.     Carbon    (except   graphite)    and   sulphur   are    hereby 

I  consumed,  the  latter  burning  with  a  blue  flame  and  odor  of 
SO2.  If  S  is  present  and  a  residue  remains,  see  if  it  will  not 
now  dissolve  in  acids. 

b.  Boil  the  residue  with  water.     This  dissolves  out  PbCl2. 

c.  Heat  the  residue  with  a  strong  NH4(C2HSO2)  solution 
acidified  with  a  few  drops  of  HC2H3O2.     Filter,  and  test  one 
portion  of  the  filtrate  for  H2SO4  with  BaCl2,  another  for  Pb 
with  an  excess  of  H2SO4,  and  a  third  for  Pb  with  (NH4)2S. 
PbSO4  is  dissolved   by  NH4(C2H3O2),  and   is  reprecipitated 
when  the  solvent  is  destroyed  by  addition  of  H2SO4. 


FOR    THE   ANALYSIS  FOR  METALS.  77 

d.  Warm   the   residue  with  water  and  a  lump 'of  KCN  > 
filter,  add  (NH4)2S  to  the  filtrate,  filter  if  a  precipitate  forms, 
wash,  dissolve  the  precipitate  in  hot  dilute  HNO3,  dilute  and 
add  HC1.     AgCl  is  dissolved  by  KCN,  and  the  Ag  is  pre- 
cipitated from  that  solution  by  (NH4)2S.     Do  not  attempt  ta 
reprecipitate  the  AgCl  by  addition  of  acids  to  the  KCN  solu- 
tion ;   for  HCN  gas,  which  is  exceedingly  poisonous,  would 
be  evolved. 

e.  If  the  residue  seems  to  be  nothing  but  SiO2,  treat  it  in 
a  platinum  crucible  with  a  few  cc.  of  HF,  evaporate  off  the 
acid  and  note  whether  anything  remains;    if  there  is  still  a 
residue,  try  to  dissolve  it  in  HC1.     In  using  HF,  take  great 
care  not  to  get  it  on  the  hands  nor  to  inhale  the  gas. 

3.  Fusion  with  Na2CO3  decomposes  most  substances,  and 
is  especially  useful  analytically  as  a  means  of  bringing  insol- 
uble  substances    into   a  soluble   form.      A  metathesis  takes 
place  ;  the  acid  radical  of  the  compound  combines  with  the 
sodium,  and  a  carbonate  of  the  metal  is  formed ;  or,  if  the 
carbonate  is  decomposed  by  heat,  the  oxide  or  the  metal  itself 
is  obtained.     The  following  are  some  examples  : 

BaSO4  -f  Na2CO3  =  Na2SO4  +  BaCO3. 
Al2Si05  +  Na2C03  =  Na2Si03  +  A12O3  +  CO2. 
4AgCl  -f  2Na2CO3  =  4NaCl  +  4Ag  +  2CO2  +  O2. 

The  acid  element  or  radical  of  the  original  compound  is- 
therefore  found  in  the  aqueous,  and  the  metallic  element  in 
the  acid  solution  of  the  fusion. 

4.  In  case  a  substance  is  dissolved  only  partially  by  acids, 
it  may  seem  simpler  to  fuse  the  original  substance  with  Na2CO3 
rather  than  the  insoluble  residue.     This  is,  however,  generally 
not  the  case ;  first,  because  some  substances,  especially  me- 
tallic sulphides  and  alkaline-earth  phosphates,  though  readily 
soluble  in  acids,  are  only  slightly  decomposed   by  Na2CO3 ;. 
secondly,  because  complete  decomposition  with  Na2CO3  is  at 
best  often  difficult  to  attain,  and  therefore  the  amount  of  sub- 
stance  treated  should  be  as  small  as  practicable ;  and  thirdly, 
because  volatile  metals,  especially  mercury,  would  be  lost. 

5.  It  is  preferable  to  use  platinum  where  admissible,  for 
two  reasons :  First,  it  is  much  easier  to  obtain  the  requisite 


7 8  PREPARATION    OF    THE    SOLUTION 

,  high 'temperature  ;  and  secondly,  Na2CO3  attacks  porcelain,  so 
that  subsequent  tests  for  aluminum  and  silica  are  unreliable. 
Platinum  vessels  cannot  be  used,  however,  when  readily  re- 
ducible metals  (Pb,  Ag,  Cu,  Hg,  As,  Sb,  Bi,  Sn),  sulphides, 
or  phosphates  together  with  organic  matter,  are  present,  as 
they  would  then  be  spoiled  through  the  formation  of  fusible 
alloys.  Alkali  hydrates  and  nitrates  in  the  fused  condition, 
and  any  solution  evolving  chlorine,  as  aqua  regia,  also  attack 
them. 

6.  The  first  and  third  reactions  of  note  3  are  examples  of 
cases    where    the    aqueous    and  acid   solutions   must   not  be 
mixed.     The  second  reaction,  a  type  of  case  very  commonly 
occurring,  admits  of  it,  however.     The  advantage  of  adding 
some  of   the  aqueous  solution  to  that  used  for  the  analysis 
for  metals  is  that  certain  metals,  those  whose  oxides  are  sol- 
uble in  excess  of  alkali,  may  pass  into  the  aqueous  solution 
as  sodium  salts. 

7.  If  the  residue  insoluble  in  acids  contains  stannic  oxide, 
calcium  fluoride,  alumina,  ferric  oxide,  or  chromic  iron,  these 
substances  would  be  little  affected  by  fusion  with  Na2CO3,  and 
would  be  found  still  insoluble  in  acids.     Therefore,  when  the 
preliminary  examination  indicates  the  presence  of  any  of  these, 
it  is  better  to  make  use  of  other  fluxes  than  Na.2CO8.     The 
choice  depends  on  the  considerations  set  forth  in  the  follow- 
ing note. 

8.  Fluxes.  —  Four  kinds  of  fluxes  may  be  distinguished : 
i.  Alkaline  fluxes,  such  as  Na2CO3,  which  are  suited  for  the  de- 
composition of  most  salts,  and  especially  of  silicates.    2.  Acid 
fluxes,  especially  KHSO4,  which  is  used  for  the  conversion  of 
basic  oxides  (A12O3  and  aluminates,  Fe2O3,  Cr2O3,  etc.)  into  sol- 
uble sulphates.     The  silica  of  a  silicate  is  lEereby  separated 
in  insoluble  form.     In  using  this  flux  take  care  not  to  con- 
tinue the  heating  till  the  sulphuric  acid  ceases  to  come  off  ; 
for  insoluble  basic  sulphates  then  result.    3.  Oxidizing  fluxes, 
for   example    mixtures   of   Na2CO3  with   NaNO3  or   KC1O3, 
which  serve  to  convert  compounds  of  acid  forming  metals  (like 
Cr,  As)  into  soluble  salts  of  their  acids.     Insoluble  native  sul- 
phides are  also  best  treated  in  this  way.     4.  Reducing  fluxes, 
for  example  a  mixture  of  Na2CO3  and  KCN,  which  are  some- 
times useful  in  separating  reducible  metals  from  their  oxides, 


FOR    THE  ANALYSIS  FOR   METALS.  79 

as  Sn  from  native  SnO2.  —  Chromic  iron  is  decomposed  by 
fusing  first  with  KHSO4,  and  then  treating  the  undecomposed 
residue  with  Na2CO3  and  NaNO3. 

Procedure.  —  To  test  for  alkali  metals  in  a  silicate  not 
acted  on  by  acids,  decompose  the  mineral  without  the  use 
of  alkaline  carbonates  as  follows  :  Mix  one  part  of  the  very 
finely  powdered  substance  with  six  parts  of  pure  CaCO3  and 
one  part  of  NH4C1,  and  heat  the  mixture  to  bright  redness 
in  a  covered  platinum  crucible  for  half  an  hour,  taking  care 
to  heat  only  the  bottom  of  the  crucible.  Heat  the  sintered 
mass  with  water  for  half  an  hour,  filter,  add  to  the  filtrate 
a  little  NH4OH,  and  (NH4)2CO3  in  slight  excess;  heat  to 
boiling,  filter,  and  remove  ammonium  salts  from  the  filtrate 
and  test  for  K  and  Na  as  directed  on  page  48. 

Notes.  —  i.  Alkali  silicates,  though  soluble  in  water  when 
alone  present,  are  often  not  dissolved  out  of  double  silicntes 
even  by  acids.  Hence  the  need  of  a  special  test  as  above 
described. 

2.  CaCO3  decomposes   the    silicate   in  the   same   way  as 
Na2CO3.     The    NH4C1    serves   to   convert   the  alkali  metals 
to   chlorides.      The    aqueous   extract   obtained  contains   the 
hydrates  and  chlorides  of  Ca,  K,  and  Na.     The  (NH4)2CO3 
is  added  to  precipitate  the  Ca. 

3.  If  the  whole  crucible  is  heated  to  bright  redness  the 
alkali   chlorides   are    lost   by   volatilization.     The   heat  used 
should  be  sufficient  to  make  the  mixture  sinter,  but  not  fuse. 

Metals  and  Alloys. 

Procedure.  —  Cut  the  alloy  into  small  pieces,  or  hammer 
it  out  so  as  to  expose  as  great  a  surface  as  possible.  Heat 
about  one  gram  of  it  with  25  cc.  of  strong  HNO3  (1.2  spec, 
grav.)  as  long  as  any  action  continues.  If  metal  protected 
from  further  action  of  the  solvent  by  the  products  of  the 
reaction  still  remains,  pour  off  the  acid,  treat  the  residue 
with  water,  and  then  with  a  fresh  portion  of  strong  HNO3. 


go  PREPARATION   OF    THE    SOLUTION 

If  complete  solution  takes  place,  dilute  with  water  and 
add  HCL  If  a  precipitate  (PbCl2  or  AgCl)  forms,  filter  it 
off  and  examine  it  in  the  usual  manner  for  Pb  and  Ag. 
Evaporate  the  filtrate,  or  the  solution  in  which  HC1  pro- 
duces no  precipitate,  to  dryness  ;  moisten  the  residue  with 
HC1,  and  evaporate  once  again  just  to  dryness  to  expel  the 
HNOo.  Dissolve  the  residue  in  water  with  addition  of  a 

o 

little  HC1,  pass  in  H2S,  and  proceed  with  the  analysis  as 
usual,  except  that  the  (NH4)2SX  treatment  of  the  H2S  pre- 
cipitate may  be  omitted.  (See  note  2.) 

If  a  white  residue  remains,  dilute  the  HNO3  solution  with 
water,  filter,  and  treat  the  filtrate  as  directed  in  the  last  para- 
graph. Wash  the  residue  with  hot  water  till  free  from  acid, 
transfer  it  to  a  porcelain  dish,  and  heat  it  with  a  consider- 
able quantity  of  (NH4)2SX.  The  white  residue  should  entirely 
dissolve ;  if  a  small  black  residue  consisting  of  sulphides  of 
metals  of  the  copper  group  remains,  filter  it  off.  Test  this 
(NH4)2SX  solution  for  As,  Sb,  and  Sn  in  the  usual  manner. 
In  case  the  white  residue  left  by  the  HNO3  is  large,  and  it  is 
found  to  be  very  difficult  to  get  it  into  solution  in  (NH4)2SX, 
treat  a  fresh  portion  of  the  alloy  with  aqua  regia,  and  pro- 
ceed with  this  solution  as  with  the  HNO3  solution,  except 
that  the  separation  with  (NH4)2SX  and  the  analysis  of  the 
tin  group  must  in  this  case  be  carried  out. 

Notes.  —  i.  The  nitrates  formed  in  dissolving  the  alloy, 
though  readily  soluble  in  water,  are  very  little  soluble  in 
HNO3;  they  may  therefore  precipitate  on  the  surface  of  the 
metal,  and  prevent  further  action  of  the  acid. 

2.  If  the  alloy  is  completely  soluble  in  HNO3,  Sn  is  absent 
and  not  more  than  a  trace  of  Sb  can  be  present.  It  is  then 
unnecessary  .to  treat  the  H2S  precipitate  with  (NH4)2SX;  foi 
of  the  metals  of  the  tin  group  only  As  and  a  trace  of  Sb  can 
be  present,  and  these  are  best  detected  by  placing  some  of 
the  solution  from  which  the  HNO3  has  been  removed  by  evap- 
oration in  a  generator  with  zinc  and  HC1.  If  present,  Sb 
and  Sn  are  oxidized  by  HNO3  to  the  white  compounds,  meta- 
stannic  acid  (H2SnO3)  and  antimony  oxide  (Sb2O3,  Sb2O4,  or 


FOR   THE  ANALYSIS  FOR  ACIDS.  81 

Sb8O5,  according  to  the  strength  of  the  acid  and  temperature). 
Traces  of  the  latter  dissolve  in  HNO3. 

3.  If  the  alloy  contains  at  the  same  time  P  or  As,  these 
elements  are  also  found  in  the  white  residue  in  the  form  of 
Sii3(PO4)4  and  Sn3(AsO4)4  respectively. 

4.  In  order  to  detect  P  in  an  alloy  completely  soluble  in 
HNO3,  it  is  only  necessary  to  add  some  of  the  solution  to  an 
excess  of  (NH4)2MoO4.     To  detect  it  in   the  white  residue, 
acidify  some  of  the  (NH4)3SX  solution  of  the  latter  with  HNO3, 
filter  out  the  precipitate,  and  add  the  filtrate  to  an  excess  of 
(NH4)2MoO4  solution. 

5.  In  the  analysis  of  an  alloy  all  the  operations  serving 
for  the  detection  or  removal  of  Cr,  Ba,  Sr,  and  Ca  are  omitted, 
since  these  metals  are  never  present.     Moreover,  K  and  Na 
need  be  tested  for  only  when  the  alloy  decomposes  hot  water. 

6.  A  black  residue  consisting  of  C  or  Si  may  remain  on 
treating   the    alloy  with    HNO3.     C  is   found   principally   in 
alloys  containing  Fe,  and  Si  in  the  newer  alloys  of  Al. 

7.  An  insoluble  residue  may  also  consist  of  the  rare  metals 
Au  and  Pt.     They  dissolve  in  aqua  regia. 

2.      FOR   THE   ANALYSIS    FOR   ACIDS. 
Salts  and  Industrial  Products. 

Procedure.  —  If  the  substance  is  soluble  in  water,  and  only 
alkaline-earth  and  alkali  metals  are  present,  dissolve  about 
one  gram,  and  use  this  aqueous  solution  for  the  various 
tests ;  if,  however,  other  metals  are  present,  add  Na^Og  to 
the  solution  as  long  as  a  precipitate  continues  to  form.  If 
the  substance  is  insoluble  and  metals  not  precipitated  by  H2S 
are  present,  boil  a  gram  of  the  finely  powdered  substance 
with  10  cc.  of  strong  Na2CO3  solution  for  ten  or  fifteen 
minutes,  replacing  the  water  which  evaporates.  Filter  off 
the  precipitate  or  residue,  and  make  half  of  the  filtrate 
slightly  acid  with  HNO3.  (If  a  precipitate  forms  when  the 
solution  is  neutral,  filter  it  off.)  Boil  this  solution  for  two 
or  three  minutes  to  expel  the  CO2,  and  use  it  for  the  BaCl2 
and  AgNOg  tests,  and  for  those  special  tests  which  may.  be 
made  in  the  presence  of  free  HNO3.  Acidify  fresh  portions 


82  PREPARATION   OF    THE     SOLUTION 

of  the  Na2CO3  solution  with  HC2H3O2  for  the  oxalic  (and 
tartaric)  acid  test,  and  with  H2SO4  for  the  nitric  acid  test. 
If  the  substance  is  insoluble,  and  only  metals  precipitated  by 
H2S  are  present,  suspend  a  gram  of  the  finely  powdered  sub- 
stance in  water,  saturate  with  H2S,  heat  to  boiling,  and  filter  ; 
boil  the  filtrate  till  the  H2S  is  completely  expelled,  and  use  it 
for  the  BaCl2  and  AgNO3  tests  and  for  the  various  special 
tests.  If  not  already  detected  or  proved  absent,  test  the 
original  substance  for  H2S  and  H2CO3. 

Notes. —  i.  All  metals  except  arsenic  and  the  alkalies  are 
precipitated  from  solution  by  Na2CO3  in  the  form  either  of 
carbonate,  basic  carbonate,  or  hydrate.  Boiling  with  Na2COs 
decomposes  almost  all  salts  more  or  less  completely,  the  acid 
radical  going  into  solution  in  combination  with  sodium,  and 
the  metal  being  precipitated  as  carbonate  or  hydrate.  Many 
of  these  carbonates  and  hydrates  dissolve  somewhat  in  the 
excess  of  Na2CO3  employed,  and  they  are  then  precipitated 
when  the  alkali  is  nearly  neutralized. 

2.  The  removal  of  the  metals  by  Na2CO3  is  necessary ;  foi 
otherwise  the  solution  cannot  be  made  alkaline,  for  example 
with  NH4OH,  without  the  production  of  a  precipitate ;  more- 
over, their  presence  interferes  in  other  ways  with  some  of  the 
special  tests. 

3.  The  Na2CO3  must  be  completely  neutralized  and  the 
CO2  expelled  before  testing  with  BaCl2  and  AgNO3 ;  other- 
wise BaCO3  and  Ag2CO3  will  precipitate. 

4.  The  removal  of  the  metals  with  H2S  where  possible  has 
the  advantages  that  it  is  more  complete  than  that  with  Na2COa, 
and  that  no  additional  salts  or  acids  are  introduced  into  the 
solution.     In  the  presence  of  chlorates  it  is  inapplicable ;  for 
the  H2S  is  then  oxidized,  and  the  tests  for  HC1O3,  HC1,  and 
H2SO4  are  valueless;     in   the    presence   of   nitrate   the   test 
for  H2SO4  is  unreliable. 

Minerals  and  Metallurgical  Products. 

Procedure.  —  Try  the  special  tests  for  H2S  and  H2CO3 
with  a  portion  of  the  finely  powdered  dry  substance.  Boil 
another  portion  with  HNO3  for  two  or  three  minutes,  ;!iiute, 


FOR    THE  ANALYSIS  FOR  ACIDS.  83 

filter  if  not  entirely  dissolved,  and  test  the  filtrate  for  H3PO4 
with  (NH4)2MoO4,  and  for  HC1  with  AgNO3. 

Test  for  H2SO4,  if  sulphides  are  absent,  by  adding  BaCl2 
to  some  of  the  HNO3  solution ;  if  the  substance  is  not  com- 
pletely soluble  in  HNO3,  test  also  the  aqueous  solution 
obtained  from  the  fusion  of  the  insoluble  residue  in  prepar- 
ing for  the  analysis  for  metals  (page  76)  by  acidifying  with 
HC1  and  adding  BaCl2.  If  sulphides  are  present,  boil  some 
of  the  original  substance  with  strong  Na^CC^  solution  for 
some  minutes,  dilute,  filter,  acidify  with  HC1,  and  add  BaCl2. 

The  presence  or  absence  of  H2SiO3  has  been  already 
determined  in  the  preliminary  examination  by  the  NaPO3 
bead  test,  or  in  the  course  of  the  analysis  for  metals. 

Test  for  HF  (with  H2SO4)  and  for  H3BO3  (with  H2SO4 
and  C2H5OH),  if  silicates  are  absent,  with  separate  portions 
of  the  original  substance.  To  test  for  these  acids,  if  sili- 
cates are  present,  fuse  at  least  a  gram  of  the  substance  with 
four  or  five  parts  of  Na2CO3,  boil  the  fused  mass  with  water, 
and  filter.  Slightly  acidify  a  little  of  the  filtrate  with  HC1, 
and  test  it  for  H3BO3  with  turmeric  paper ;  evaporate  half  of 
the  remaining  filtrate  to  dryness,  and  test  for  H3BO3  with 
H2SO4  and  C2H5OH.  Acidify  the  other  half  of  the  filtrate 
with  HC2H3O2,  allow  it  to  stand,  filter  off  the  precipitate, 
add  CaCl2,  allow  the  liquid  to  stand,  collect  the  precipitate 
on  a  filter,  and  test  it  in  the  usual  manner  for  HF. 

Notes.  —  i.  Boiling  with  Na2CO»  fails  to  decompose  many 
insoluble  minerals,  slags,  etc.  Moreover,  it  is  necessary  to  test 
these  substances  for  only  a  comparatively  small  number  of 
acids;  namely,  for  H2S,  H2CO3,  H2SiO3,  H2SO4,  H3PO4. 
H3BO3,  HF,  and  HC1 ;  for  the  other  acids  do  not  occur  in 
them,  or  occur  very  rarely.  For  these  reasons  a  different 
method  of  treatment  has  been  given. 

2.  H2S  and  H2CO3,  if  present,  are  usually  detected  at  the 
beginning  of  the  analysis  on  treating  the  substance  with  acids. 
It  is  only  for  traces  that  it  is  sometimes  necessary  to  test  more 
carefully  here. 

3.  Many  phosphates  are  decomposed  very  incompletely  by 


84  PREPARATION  OF  THE  SOLUTION. 

boiling,  and  even  by  fusing,  with  Na2CO3 ;  but  they  all  dissolve 
without  difficulty  in  HNO3,  and  are  therefore  best  tested  for 
in  this  solution. 

4.  All  chlorides  except  AgCl  can  also  be  tested  for  in  the 
HNO3   solution.     If   Ag  has  been  found  among  the  metals, 
the  solution  obtained  by  boiling  or  fusing  with  Na2CO3  must 
also  be  tested  for  HC1. 

5.  Sulphides  are  partially  oxidized  to   sulphates  both  by 
boiling  with  HNO3  and  by  fusing  with  Na2CO3.     Therefore 
the  substance  is  boiled  with  Na2CO3,  or,  if  completely  soluble, 
a  HC1  solution  of  it  may  be  prepared,  in  order   to  test  for 
H2SO4  in  the  presence  of  H2S. 

6.  In    the  presence  of  silicates  the  tests  for  H3BO3  and 
HF,  if  made  with  the  original  substance,  would  be  unreliable, 
because  the  mineral  may  not  be  decomposed  by  H2SO4 ;  and 
in  the  case  of  HF,  also  because  this  acid,  if  set  free,  may  be 
entirely  converted  to  SiF4  by  the  silica  of  the  mineral. 

7.  By  fusing  such  a  mineral,  Na2SiO3,  NaBO2,  and  NaF 
are  obtained  in  solution.     Silicic  acid  is  precipitated  on  acid- 
ifying with  HC2H8O8,  and  CaF2  on  the  addition  of  CaCls. 


.APPENDIX. 


PREPARATION     OF    REAGENTS. 


ACIDS.  n 

Acetic :  sp.  gr.  1.044 ;  m*x  °ne  part  of  the  glacial  acid  with 
2^  parts  of  water. 

Aqua  regia :  mix  i  part  HNO3  with  three  parts  of  con- 
centrated  HC1. 

Hydrochloric,  concentrated  :  sp.gr.  1.12. 

Hydrochloric,  dilute  :  mix  one  part  of  the  concentrated 
acid  with  4  parts  of  water. 

Hydrochloroplatinic  :  100  grams  H2PtCl6.6H2O  to  the  liter, 

Hydrogen  sulphide  :  saturated  solution. 

Phenolsulphonic:  dissolve  150  grams  of  phenol  in  600 
grams  of  concentrated  H2SO4. 

Sulphuric,  concentrated:  sp.  gr.  1.84. 

Sulphuric,  dilute  :  mix  i  part  of  the  concentrated  acid  with 
4  parts  of  water. 

Sulphurous  :  saturated  solution. 
rrtoo* 

AMMONIUM  SALTS. 

Acetate  :  add  1000  cc.  NH4OH  (sp.  gr.  0.90)  to  1250  cc. 
glacial  acetic  acid. 

Carbonate  :  dissolve  250  grams  in  a  liter  of  water  and  add 
100  cc.  NH4OH  (sp.  gr.  0.90.) 

Chloride  :  100  grams  to  the  liter. 

Hydroxide  :  sp.  gr.  0.96  ;  mix  i  part  NH4OH  (sp.  gr.  0.90) 
with  2  parts  water. 

Molybdate  :  mix  100  grams  MoO3  with  400  cc.  cold  dis- 
tilled  water  and  add  80  cc.  of  ammonia  (0.90  sp.  gr.).  Filter, 
and  pour  this  solution  with  constant  stirring  into  a  mixture  of 
300  cc.  HNO3  (1.42  sp.  gr.)  and  700  cc.  water. 

Oxalate  :  40  grams  (NH4)2C2O4,2H2O  to  the  liter. 

Sulphide  (colorless)  :  saturate  1500  cc.  NH4OH  (sp.  gr.  0.90) 
with  H2S  ;  then  add  1000  cc.  NH4OH  (sp.  gr.  0.90)  and  2000  cc. 
of  water. 

Persulphide  (yellow)  :  add  50-75  grams  of  sulphur  to  col- 
orless (NH4)2S. 

Sulphate  1250  grams  to  the  liter. 


86  APPENDIX. 

BARIUM  SALTS. 

Chloride  :  20  grams  BaCI2.2H2O  to  the  liter. 
Hydroxide  :  50  grams  BaO2H2.8H2O  to  the  liter. 
CALCIUM  chloride  :  100  grams  CaCl2.6H2O  to  the  liter. 
FERRIC  chloride  :  100  grams  FeCl3  to  the  liter. 
FERROUS  sulphate:  dissolve  200  grams  FeSO4.7H2O  in  a  liter  of 
water;  place  scraps  of  iron  in  the  solution,  and  add  a  few 
drops  of  H2SO4  from  time  to  time. 
LEAD  acetate  :  100  grams  Pb(C2H3O2)2.3H2O  to  the  liter. 
MAGNESIUM  ammonium  chloride :  dissolve  90  grams  MgCl2.6H2O 
and  240  grams  NH4C1  in  a  liter  of  water,   and  add  50  cc, 
NH4OH  (sp.  gr.  0.90)  to  the  solution. 
MERCURIC  chloride  :  50  grams  to  the  liter. 
POTASSIUM  SALTS. 

Acetate  :  saturated  solution. 
Chromate  :  100  grams  to  the  liter. 
Ferricyanide  :  10  grams  to  the  liter. 
Ferrocyanide  :  15  grams  to  the  liter. 
Nitrite:  500  grams  to  the  liter. 
Sulphocyanate :  100  grams  to  the  liter. 
SILVER  SALTS. 

Nitrate  :  25  grams  to  the  liter. 
Sulphate :  saturated  solution. 
SODIUM  SALTS. 

Acetate  :  saturated  solution. 

Carbonate  :  150  grams  of  the  anhydrous  salt  to  the  liter. 
Hydroxide  :  100  grams  (Solvay)  NaOH  to  the  liter. 
Hypochlorite  :  add  500  grams  of  bleaching  powder  and  245 
grams  of  anhydrous  Na2CO3  to  4000  cc.  of  water ;  allow  the 
precipitate  to  settle  and  decant  the  solution. 

Phosphate  :   100  grams  Na2HPO4.i2H2O  to  the  liter. 
Sulphide  :  saturate  a  10  per  cent.  NaOH  solution  with  H2S, 
and  dissolve  50  grams  of  sulphur  in  i  liter  of  the  solution. 
STANNOUS  chloride  :  heat  an  excess  of  granulated  tin  with  concen- 
trated HC1,  adding  scrap  platinum  to  facilitate  the  solution. 
Dilute  with  an  equal  volume  of  water,  and  keep  the  solution 
in  well-stoppered  bottles  containing  metallic  tin. 


INDEX. 


ACETATES,  detection,  54,  63 

solubility,  50 
Acids,  detection,  50 

detection  in  salts,  81 

detection  in  minerals,  82 

division  into  groups,  51 

general  tests,  52,  53 
Alkalies,  precipitation   of   metals  by, 

23>  3°'  43 

Alkali-group,  detection  in  silicates,  79 
separation,  48 
carbonates,  occurrence,  32 
carbonates,  properties,  46 
Alkaline-earth  group,  separation,  46 
group  separation  from  iron  group, 

3r>  40 

nitrate-s,  properties,  46 
sulphates,  properties,  40 
Alkaline    solutions,    precipitation    by 

acid,  17 

Alloys,  analysis,  79 
Aluminum,  detection,  37,  42 
Aluminum  group,  behavior  with  alka- 
lies, 30,  43 
precipitation,  31 
preliminary  tests,  29 
separation,  36,  38 
separation  fr<>m  copper  group,  17 
Aluminum  hydroxide,  occurrence,  19, 

22,  38 

hvdroxide,  properties,  30,  44,  48 
phosphate,  occurrence,  48 
Ammonium,  detection,  49,  67 

magnesium   arseniate,  properties, 

25,  56 
magnesium  phosphate,  properties, 

43,  48,  56 

phosphomolybdate,  properties,  55 
platinichloride,  properties,  48 
salts,  solubility,  50 
salts,  solvent  action,  22,  31,  34,  46, 

52,  64 

Antimony,  detection,  25,  67 
hydride,  properties,  26 
oxide,  occurrence,  21 
oxychloride,  occurrence,  17,  18 
sulphide,  properties,  18,  19 


Arseniates  and'  arsenites  distinguished, 
26 

solubility,  50 
Arsenic,  detection,  24,  25,  67,  68 

detection  in  paper,  fabrics,  etc.,  74 

hydride,  properties,  26 

sulphide,  properties,  18,  26 
Arsenious  sulphide,  properties,  18,  19 


BARIUM,  detection,  40,  46,  69,  70 

chloride,  occurrence,  17 

chromate,  properties,  47,  52 
Bismuth,  detection,  22,  68 

oxychloride,  occurrence,  i/,  18 

oxychloride,  properties,  22 

sulphide,  occurrence,  20 

sulphide,  properties,  18 
Bivalent    and   trivalent  metals   sepa- 
rated, 41 

Blowpipe  reactions,  33,  68 
Borates,  detection,  56,  83 

of  alkaline  earths,  occurrence,  35 

solubility,  50 
Borax  bead  test,  69 
Bromides,  detection,  54,  58,  67 

detection  in  presence  of  iodides,  ^o 

solubility,  50 


CADMIUM,  detection,  22,  68 

ferrocyanide,  properties,  23 

sulphide,  properties,  18,  23 
Calcium,  detection,  40,  46,  70 

tartrate,  properties,  64 
Carbonates,  detection,  54,  57,  73 

solubility,  50 

Chlorates,  as  oxidizing  agents,  18,  24, 
29 

decomposition  by  ignition,  62,  63, 
68 

detection,  54,  62,  67,  73 

solubility,  50 
Chlorides,  detection,  54,  58,  67,  84 

detection   in    presence    of    oth^r 
halogens,  60 

solubility,  16,  17,  50 


88 


INDEX. 


Chromates,  as  oxidizing  agents,  18,  29 

detection,  55,  68,  73 
Chromic  iron,  decomposition,  79 
Chromium,  detection,  42,  69 

effect  on  iron-group  separation,  35 

hydroxide,  properties,  30,  32,  37 

oxychloride,  properties,  61 
Closed  tube  test,  66 
Cobalt,  detection.  33.  68,  69 

hvdroxide,  properties,  34 

occurrence,  37,  43 

sulphide,  properties,  30,  33 
Copper,  detection,  22,  68,  09,  70 

ferrocyanide,  properties,  22 

occurrence.  20 

sulphide,  properties,  18,  20 
Copper-group,  behavior  with  alkalies, 

23 

precipitation,  17 
separation,  20 
separation  from  other  groups,  16, 

'9 

sulphides,  occurrence,  33 

sulphides,  properties,  20 

Cyanides,  detection,  54,  59,  67,  73 

DRY  analysis,  65 

FERRIC  and  ferrous  salts  distinguished, 

44 

ferrocyanide,  properties,  37 
hydroxide,  occurrence,  19,  22 
hydroxide,  properties,  30,  31 
salts,  as  oxidizing  agents,  18 

Ferricyanides,  detection,  59 

Ferrocyan  des,  detection,  59 

Ferrous   sulphide,  occurrence,  23,  33, 

'  38,  43 

properties,  30 
Flame  tests.  40,  48,  70 
Fluorides,  detection,  54,  57,  83 

effect  on  iron-group  separation,  35 
Fluxes,  properties,  78 
Fusion,  decomposition  by,  77,  78 

HALOGEN-ACID-GROUP,  detection,  53 

special  tests,  58 
Hydroxides,  colors,  23,  30 

solubility,  23,  30,  31,  43 

IMPURITIES  in  reagents,  14 
Incrustations  on  charcoal,  68 
Insoluble  substances,  74,  76 

substances,  treatment,  75,  76,  80 
Iodides,  detection,  54,  59,  67 

solubility,  50 
Iron  (see  also  "  ferric  and  ferrous  ") 

detection,  37,  40,  68,  69 


Iron-group,  behavior  with  alkalies,  43 
precipitation,  31 
preliminary  tests,  29 
separation,  36,  38 
separation  from  copper  group,  17 
sulphides,  properties,  30,  32 

L.EAD,  detection,  17,  21,  68 

chloride,  properties,  16,  17 
chromate,  properties,  42,  55 
sulphate,  occurrence,  20,  23,  38 
sulphate,  properties,  18,  21 

Litmus,  significance  of  reaction  with, 
73 

MAGNESIUM,  detection,  43,  47 

ammonium  phosphate,  properties, 

43.  48,  56 

hydroxide,  properties,  31 
removal,  48 

Manganese,  detection,  37,  43,  69 
hydroxide,  properties,  31 
sulph  de,  properties,  30 

Mercuric   and  mercurous  salts  distin- 
guished, 23 
oxide,  properties,  23 
sulph'de,  occurrence,  23,  67 
sulphide,  properties,  18,  20,  21, 

Mercurous  chloride,  properties,  17,  25, 

27 

oxide,  properties,  23 
Mercury,  detection,  17,  21,  23,  67 
Metals,  division  into  groups,  15 

reducible  on  charcoal,  68 

removal  from  solutions,  48,  82 
Metaphosphate  bead  test,  69 
Minerals,  detection  of  acids  in,  82 

solution  for  analysis,  72 

NICKEL,  detection,  33,  68,  69 
hydroxide,  properties,  34 
occurrence,  37,  43 

Nitrates,  as  oxidizing  agents,  18,  29 
decomposition  by  heating,  63 
detection,  54,  61,  62,  63,  67,  68 

Nitric-acid-group,  properties,  52 
special  tests,  61 

ORGANIC  acids,  special  tests,  63 
matter,  detection,  51,  66 
matter,  removal,  74 
matter,  solvent  action,  30 

Oxalates,  detection,  54,  56,  67 

of  alkaline  earths,  occurrence,  35 

Oxidation  and  reduction,  27,  28 
of  ferrous  salts,  28,  30 
of  sulphides,  18,  21,  32,  33 

Oxidizing  agents.  29 


INDEX. 


PERMANGANATES,  properties,  29,  44 
Phosphates,  detection,  54,  55,  83 

effect  on  iron  group  separation,  35 

of  al kaline  earths,  occurrence,3O,34 

solubility,  50 

Phosphorous,  detection  in  alloys,  81 
Platinum,  in  alloys,  81 

vessels,  use  of,  77 
Potassium,  detection,  48,  70 

acid  tartrate,  properties,  50,  64 

platinichloride,  properties,  48 

removal,  49 

salts,  solubility,  50,  64 
Precipitation,  directions  for,  13 

by  water,  22 
Preliminary  examination,  65 

REACTION  towards  litmus,  73 
Reactions    of    oxidation,   method    of 

writing,  28 

Reagents,  preparation,  85 
Reducing  agents,  29 
Reduction  and  oxidation,  27*  28 

of  ferric  salts,  30 

on  charcoal,  68 

SILICATES,  detection,  54,  58 

detection  of  alkalies  in,  79 
Silicic-acid,  bead  test,  69 

occurrence,  17,  38,  73 

removal,  73 

solubility,  58 

Silicon  fluoride,  properties,  57 
Silver,  detection,  17,  68 

chloride,  properties,  17,  6l,  77 

iodide,  properties,  61 

salts,  properties,  53 

sulphide,  properties,  18 
Sodium,  detection,  48,  70 

chloride,  occurrence,  17 

platinichloride,  properties,  49 

salts,  solubility,  50 
Solubility,  general  statement  of,  50 


Solution  of  nonmetallic  substances,  72 

of  metals  and  alloys,  79 

for  analysis  for  acids,  81,  82 
Spectrum  analysis,  49,  71 
Stannic    and    stannous     salts    distin- 
guished, 27 

oxide,  occurrence,  21 

sulphide,  occurrence,  25 

sulphide,  properties,  18,  24 
Strontium,  detection,  40,  46,  69,  70 

sulphate,  properties,  47 
Sublimates  in  closed  tube,  67 
Sulphates,  detection,  52,  55,  83 

solubility,  47,  50 
Sulphides,  as  reducing  agents,  18,  29, 

colors  of,  18,  30 

detection,  54,  60,  67,  68,  73 
Sulphites,  detection,  54,  67,  73 
Sulphur,  occurrence,  i8,.i9,  31,36,44, 

74 

Sulphuric-acid  group,  detection,  52,  53 
group,  special  test,  55 
test  for  acids,  53 

TARTRATES,  detection,  54,  63 
Thiosulphates,  detection,  54 
Tin  (see  also  "  stannic  ") 

.  detection,  25,  68 
Tin-group,  behavior  with  nitric  acid,  80 

precipitation,  i" 

separation,  24 

separation  from  copper  group,  19 

WASHING  precipitates,  directions  for, 

14 

Water,  precipitation  by,  18,  22 
detection,  66 

ZINC,  detection,  37,  43,  68 
hydroxide,  properties,  38 
occurrence,  18,  36 
sulphide,  properties,  30 


'  J. 


</'• 


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